Calvon Reid writhed in pain before he died. “I can’t breathe. I can’t breathe,” the 39-year-old screamed. Reid, an African American father of two, was held face-down by two police officers on a grassy lawn inside a predominantly white, gated retirement village in the south Florida suburb of Coconut Creek in the early hours of 22 February.

Moments before, two officers, standing 10ft away, had deployed their weapons in tandem. Not guns, but Tasers. The barbs struck Reid in the chest, according to eyewitnesses, unleashing 50,000-volt shocks to his body. Reid stopped breathing within moments; two days later, he died in the hospital.

“The whole thing seemed brutal,” 58-year-old locksmith John Arnendale told the Guardian from the ground-floor apartment at Wynmoor Village retirement homes where he watched Reid lose consciousness for the last time.

It is not clear why the officers were trying to arrest Reid in the first place. He was not accused of any crime. Though police say he was acting aggressively, other witnesses have disputed this.

“They didn’t have to use a Taser to stop him,” Arnendale said. “There were four of them and he wasn’t huge or particularly athletic. They certainly didn’t choose the least harsh thing to do with him. They were kind of punishing him.”

While deadly police shootings in the United States have gained international attention this year, Reid is one of 47 lesser-known people who lost their lives after law enforcement officers deployed a Taser, according to The Counted, an ongoing Guardian investigation documenting fatalities that follow police encounters.

Reid’s case is, in many ways, tragically typical of the other deaths following the use of a Taser by police in 2015: he was unarmed, as in all but three cases. Like nearly 40% of the victims, he was black. And as in at least 53% of such cases, the suspect was displaying signs of intoxication before his or her death. As with many of these incidents, Reid died following shocks administered seemingly in violation of national guidelines, by officers belonging to a police department with lax rules on how these less-lethal weapons should be used.

As Tasers became an increasingly prevalent part of police officers’ arsenals around the world, the US Justice Department funded the Police Executive Research Forum (Perf), an independent policing thinktank, to revise guidelines on their use in 2011. These rules are designed to encourage officers to know Tasers “should not be seen as an all-purpose weapon that takes the place of de-escalation techniques” – and to acknowledge the lethal potential of electronic control weapons (ECW) deployed for more than three standard shock cycles of five seconds each.

“When Tasers were first introduced, it was thought they really could be used without causing any harm,” Perf executive director Chuck Wexler said in a phone interview. “Subsequently, in our research and work, we realised that extended use of ECWs could cause injuries and death. That is why we stipulate restrictions on their use.”

The Guardian can now reveal that many police departments are still not regulating the use of Tasers in accordance with these nationally accepted standards. According to 29 guidebooks on ECWs obtained by the Guardian from police departments where a death has occurred after the weapon was deployed this year, an overwhelming majority of them flout key tenets of the expert advice:

Twenty police departments do not guide officers against more than three shocks in all but exceptional circumstances.

None of the 29 departments, according to their use of force guidelines, mandate use-of-force investigations into incidents where an ECW is deployed for more than 15 seconds.

Twenty-two departments do not advise against deploying ECWs if the sole justification is that the suspect is fleeing.

Twenty-five departments do not advise against using an ECW’s “drive stun” mode, when the weapon is thrust directly into the skin to cause pain, as a compliance technique.

Thirteen departments do not explicitly restrict officers from deploying their ECWs if a suspect is already in handcuffs and does not pose an exceptional threat.

Eight departments do not even explicitly require officers to give a warning, when possible, before the ECW is deployed.

“One of the key challenges in American policing is that you have 18,000 police agencies,” said Wexler, frustrated. “So when we put forward our guidelines, we can only emphasise that departments consider them.”

Indeed, all of these guidelines were seemingly overlooked in the case of Calvon “Andre” Reid.

“Not only should they not have been using the Tasers,” said attorney Jarrett Blakeley, who is representing Reid’s family in a suit against Coconut Creek and its police department, “clearly they were using the Tasers incorrectly.”

But Reid’s death was also exceptional in that Taser shocks were explicitly found to be the cause.

While the pathologist in Coconut Creek determined that Reid had cocaine and alcohol in his blood and had a predisposed heart condition, his death was ruled a homicide caused directly by an ECW.

One reason deaths that follow Taser use may be attracting less of the spotlight is the difficulty pathologists often have in assessing the weapon’s role, if any, in how a person dies. When someone is shot with bullets, the cause of death is usually unquestionable. But in deaths following a Taser shock, there is often a complicated mixture of circumstances, and in many cases it is drug use, or the deceased person’s resistance to arrest, that is determined to be the cause.

Of the 47 officer-involved deaths that have occurred following the use of a Taser this year, the Guardian has obtained 19 rulings by medical examiners. Seven have been declared homicides, with five ruled “undetermined”, six “accidental” and one attributed to “natural causes”. Although police activity, often directly acknowledging the ECW, was ruled a factor in 13 of the 19 cases, only in Reid’s was the ECW determined to be the primary cause of death.

Taser International, which sells ECWs to 17,800 of the United States’ roughly 18,000 law enforcement agencies and commands an overwhelming monopoly on the market, has said their weapons do not kill. The billion-dollar company has also sued medical examiners in the past, in one case leading to the examiners’ representative body to state that Taser International’s actions were “dangerously close to intimidation”.

“A Taser exposure is not risk-free,” a company spokesman said in an email response to a detailed list of questions for this article.

But the company’s position increasingly flies in the face of a growing medical argument to the contrary, as researchers insist that under certain circumstances, however rare, Taser shocks can lead directly to a person’s death.

These medical professionals argue the lethal potential of Tasers is being underestimated – partly thanks to an “aggressive” push by Taser International to fund research of its own – and that the weapons are likely responsible for many more deaths than coroners can easily record.

Science takes on real-life circumstances at heart of Taser debate

The precise circumstances of why Reid was present in the parking lot of that south Florida retirement home in February remain unclear. Eyewitnesses quoted in a recently filed lawsuit against the officers, however, claim he was neither aggressive nor committing any crime before police arrived. He had approached a resident asking for medical help, but when an ambulance arrived, Reid declined their assistance. It was at that point when police were called and the situation escalated.

By the time John and Bonnie Arnendale awoke the next morning, they said all signs of altercation had disappeared, police tape had been removed, and Taser wires were no longer on the ground.

“It was like nothing had ever happened. It was just peaceful and beautiful. It almost felt dream-like,” said Bonnie Arnendale. “Or nightmare-like.”

The Coconut Creek police department took almost two weeks to publicly acknowledge Reid’s death and did not interview the Arnendales until the story broke in local media the following week. Two months later, all four officers were cleared of any wrongdoing in an internal investigation, but a criminal investigation by the county attorney remains open.

“We believe that if they had not used the Tasers, our son would be alive today,” Calvon’s mother, Mamie Reid, said from her South Carolina home. “Why was he treated so inhumanely? Why in America would cops treat people in that manner?”

The Coconut Creek police department declined all interview requests from the Guardian. But some information has slowly seeped out: three of the four officers involved in the incident did not have up-to-date certification for using a Taser – a violation of Florida state law and internal policy. Reid was seemingly shot in the chest and shocked by multiple weapons simultaneously. One month after the incident, Coconut Creek police chief Michael Mann was forced to resign.

***

There is little dispute that ECW use, under appropriate conditions, is both effective and safe. A 2011 Justice Department survey of 12 police departments using Tasers found that deploying an ECW over other comparable use-of-force tactics, such as physical force, reduced the odds of a suspect’s injury by 60%. In the same year, the Justice Department found that the risk of death in an ECW-related incident was “less than 0.25%”, adding that in the “large majority” of even these cases the weapons “do not cause or contribute to death”.

In 2009, Taser International noted that of more than 650,000 Taser exposures among law-enforcement volunteers, none reported significant effects on their heart rhythms. Taser International told the Guardian that this number now stood at more than 2m voluntary exposures.

But critics point to the reality that these exposures – typically a short, single-weapon shock delivered to the back of a healthy police officer – do not reflect the circumstances of many ECW applications.

Numerous studies, such as one published in the Annals of Emergency Medicine in August 2011, point out that the majority of subjects exposed to Tasers in the field were either “under the influence of alcohol or illicit drugs, or had psychiatric co-morbidities”. And, as in many of the cases where a suspect has died in 2015, field situations also frequently involve multiple discharges.

Experts argue that all these factors dramatically increase the chances that an ECW exposure could be followed shortly by death.

The discussion is based in part around a principle cardiologists refer to as “capture”. Since the heart uses electrical impulses to coordinate its beating rhythm, electricity from sources outside the body have the capacity to “capture” the heart and alter its beat. This is the same principle behind defibrillation, when doctors use electrified paddles to shock a patient’s heart back into a normal beat when its rhythm has become too irregular to properly pump blood.

When a heart is beating correctly, however, electricity can instead disrupt the impulses from its internal pacemaker and cause the heart to enter into a dangerous and irregular rhythm known as ventricular fibrillation, or VF. Left unaddressed, VF rapidly leads to cardiac arrest and then death. And unlike defibrillators, which will typically try to pace a heart at 70 beats a minute, a standard Taser X26P pulses at a rate of 19 shocks a second, or 1,140 a minute.

According to Dr Douglas Zipes, a cardiac electrophysiologist and professor of medicine at Indiana University, that presents a problem. “The normal heart cannot withstand such rapid rates,” he said. “When a normal heart rate stimulated by electricity exceeds 250 times a minute, the entire conduction system breaks down and the heart goes into cardiac arrest.”

Laboratory research remains split on whether the high-voltage, low-current shock delivered by an ECW is capable of triggering the irregular rhythm of VF. For ethical reasons, many available studies on the potential cardiac effects of Taser charges have been conducted on anesthetized animals – often pigs, whose hearts are biologically similar to humans.

“The animal studies have been the underpinning of understanding what can happen, however they can only go so far,” Zipes said, noting physiological differences between pigs and humans, as well as the difficulty in recreating the variables of a Taser deployment in the field.

In 2012, Zipes published a report looking at eight cases when a person who had been Tasered in the chest lost consciousness “during or immediately after” being shocked by police. In all eight incidents, EKG readings show either VF, ventricular tachycardia (VT) or asystole, which is the medical term for a flatline reading with no detectable heart rhythm. Zipes concluded that “cardiac arrest due to VF can result from an ECD shock”, referring to an alternative name for electronic control weapons. He also concluded that law enforcement officers “should be judicious how and when to use the ECD weapon, [and] avoid chest shocks if possible”.

Steve Tuttle, the Taser International spokesman, said Zipes’s report is “based on uncontrolled and anecdotal observations”, and noted in an email to the Guardian that it “does not prove a cause-effect association”. He pointed to a 2012 Wake Forest study funded by the National Institute of Justice which found that out of 178 field chest strikes with Taser weapons, no subjects suffered any heart-related complications.

But in the case of 18-year-old Israel Hernandez-Llach, a graffiti artist from Miami who died after being shocked in the chest in August 2013, the pathologist ruled the teenager died from “sudden cardiac death” as a result of the shock. Hernandez-Llach had fled from Miami Dade police officers after painting on an abandoned building. Nonetheless, his death was ruled accidental. Prosecutors, who did not charge officer Jorge Mercado over the death, argued the officer did not intend to kill the teen as he did not know the weapon could be lethal.

Although a records request for the full autopsy in the death of Calvon Reid – who was, according to eyewitnesses, shocked in the chest with two Tasers – was rejected pending an ongoing criminal investigation, a medical source with intimate knowledge of the case told the Guardian that the pathologists involved “stood by what is on the death certificate”.

“This case was done very, very well. It’s a well thought-out opinion in cause and manner of death,” the source said, before declining to elaborate.

Taser International strongly denies the suggestion that its weapons are capable of triggering VT, a rapid heart rate that can sometimes precede the irregular and potentially fatal rhythms of VF. Still, in 2009 the company updated its recommended targeting guide to chest shots, in order to avoid “the controversy about whether ECDs do or do not affect the human heart”.

Mamie Reid remains unconvinced. “I would like for them to have three cops – Tasing them at the same time, in the chest, see if they could take that test,” she said. “Then maybe we could all be believers.”

Only 12 of the 29 police department rulebooks obtained by the Guardian from around the country explicitly advise against shocks to the chest.

Death by Taser: a difficult determination

Chance Thompson was striding and shadowboxing when two Yuba County deputies encountered him on top of a wall outside a deserted manufacturing plant in rural California. It was the early hours of 15 February, and both deputies, Jaime Knacke and Daniel Trumm, observed that Thompson appeared to be high on drugs.

They instructed him to get down, but when Thompson seemed oblivious to their requests, Trumm grabbed him by the trousers and pulled him off the ledge. A passerby captured part of the altercation on video, showing the two grappling briefly before Knacke deployed her Taser. Thompson fell to the ground instantly.

The video shows that Trumm is seemingly able to gain control of the 35-year-old, who was flat on his face after the Taser is fired. But the eyewitnesses stopped recording shortly after. According to the deputies’ account, Thompson subsequently “bucked” them both off and continued resisting.

The data from Knacke’s Taser, revealed in documents released to the Guardian by the county prosecutor, shows Thompson was subsequently shocked another five times – each for a full five-second cycle – before he was placed in handcuffs. He was then shocked once more, meaning a total of seven cycles – or 35 seconds.

It was at this point, according to their report, when the deputies noticed Thompson was having difficulty breathing. He had “white foam around his mouth and nose, his face was red, and his pupils were fully dilated with no iris color visible,” they observed. Shortly after that, he lapsed out of consciousness and his heart stopped beating. Despite later being revived, Thompson was pronounced brain-dead in hospital. The life-support machine was switched off two days later.

“What Chance was doing was non-violent. Nobody was in danger from him – he was out in the middle of the country,” Thompson’s stepfather, Ray Guthrie, said in a telephone interview. “You Taser him once, and he’s on his back, with the officer’s knees in him. He’s not resisting arrest anymore. But they just continued.”

The Yuba County sheriff’s department has one of the more restrictive policies obtained by the Guardian, advising that deputies should acknowledge signs of intoxication before use, refrain from deploying Tasers on people in restraints, and avoid shocks in the chest. But both internal and criminal investigations found no wrongdoing.

Thompson’s autopsy, conducted two days after the altercation and obtained under a records request, classed the cause of his death as a lack of oxygen to the brain following cardiac arrest after a “violent struggle”. While the pathologist noted the “attempted restraint with electronic control device” as a significant condition, the manner of death was left “undetermined”. The Taser darts were noted in Thompson’s lower back and left shoulder.

For Yuba County district attorney Patrick McGrath, this medical ruling appeared to be enough to absolve the officers of criminal responsibility. “I’m not even sure [the Taser] contributed,” McGrath told the local newspaper. Noting the high levels of methamphetamine found in Thompson’s blood (the 35-year-old had a long history of substance abuse), McGrath continued: “His methamphetamine level … was off the charts.”

Thompson’s family considers that line of argument part of a cover-up. “It was quite obviously excessive use of force,” said Guthrie. “He wouldn’t have died if they hadn’t have tased him several times. Obviously.”

•••

Other studies on the lethal potential of ECWs have centred on the question of whether multiple shocks – irrespective of placement – can contribute to a cardiac arrest.

Dr Zian Tseng, a cardiologist and electrophysiologist at the University of California San Francisco, argues that ECW shocks can cause death by what he calls the “indirect method”, where the “intense pain and adrenaline surge can indirectly induce cardiac arrest”.

Much of the argument relies on a process called metabolic acidosis. Since ECWs cause muscle tissue to rapidly contract, they also force muscles to produce the byproduct of contraction: lactic acid. This is the same substance that causes muscle soreness hours or days after a strenuous workout. When lactic acid rapidly enters the bloodstream at a speed faster than the kidneys can process, it lowers the pH of the blood, which can increase the risk of a fatal arrhythmia or cardiac arrest. Single Taser shocks produce little lactic acid, but prolonged use increases the amount. It is believed that illicit drugs in a person’s system can further add to this risk.

In 2008, a California jury found that Taser International “knew or should have known that prolonged administration of electricity from the devices pose a danger, i.e., a risk of acidosis to a degree which posed a risk of cardiac arrest”.

The finding came in a civil lawsuit against Taser International by the family of 40-year-old Robert Heston, who died after receiving at least 25 shocks over a 74-second span. The medical examiner in Heston’s case ruled his death was due to cardiac arrest caused by his “agitated state associated with methamphetamine intoxication and applications of Taser”. The jury declared Taser International 15% responsible for Heston’s death, and Heston himself 85% to blame.

Tuttle, the Taser International spokesman, argued that “human studies” on the effects of acidosis “consistently show no interference with breathing”.

Part of the difficulty in parsing studies on the potential medical effects of Taser use is the outsize influence of Taser International in funding research. Several of the more prolific researchers on the subject were either partially funded by Taser International or forged official relationships shortly thereafter.

For example, Jeffrey Ho, who has authored dozens of papers on the safety of Tasers, was named the company’s medical director in 2009 and owns shares in the company. Another prominent researcher, Mark Kroll, who co-authored a 2009 book on Taser weapons with Ho, also serves as a board member and scientific adviser for Taser International. According to the company’s 2014 annual report, Kroll owned 34,130 shares of Taser International as of 17 March, at a value of just over $800,000 according to the share price of $23.44 for that day. (Ho and Kroll did not respond to interview requests from the Guardian.)

One 2011 study, co-authored by Dr Tseng, found that research funded by Taser International on the potential risks of its own devices was nearly 18 times more likely to conclude that a Taser is safe than independent research.

Tseng told the Guardian that in 2005, after he was quoted in a San Francisco Chronicle article suggesting that Tasers could induce cardiac arrhythmia, the company asked him to reconsider his position and offered him grant money for new research.

“They were very, very aggressive with me in my early career,” Tseng said.

Tuttle argued that the weapons are “the most studied less lethal tool on an officer’s belt”. He stated in an email that of more than 700 “safety studies, human studies, abstracts, reports, letters, etc”, 78% were “independent of Taser International”.

•••

Cases like Chance Thompson’s underline the difficult position pathologists often find themselves in when assessing the role an ECW may have played in a death.

“It’s essentially impossible to make that assessment through an objective examination of the body itself,” said professor Marcus Nashelsky, president of the National Association of Medical Examiners and a professor of pathology at the University of Iowa. “As important is to have a very clear, sequential understanding of what was occurring during this event immediately before, during and after use of the ECW.

“It’s the information about circumstances, and changes in the decedent around the time of the use of the ECW, that provides extremely valuable information to the medical examiner.”

Simply put: if an ECW is likely to contribute to or cause a death, it is expected that a suspect will go into distress shortly after the shocks are administered. But determining whether this occurred can often mean medical professionals are relying on a police officer’s account of the incident.

In Thompson’s case, the Yuba County coroner’s office – which is part of the sheriff’s department – confirmed that the eyewitness video was not reviewed by the pathologist who made the determination. Instead, the deputies’ own report of the event was used to determine the course of events.

In the case of Terrance Moxley, a 29-year-old African American from Mansfield, Ohio, who was staying at a halfway house following his release from prison after a drug-dealing conviction, even video evidence was not enough to assess the effects of multiple Taser shots to the chest.

Moxley, who had been violently tripping on synthetic marijuana on 10 March before police were called, was carried to a cruiser in handcuffs by a group of five officers. He managed to break free of the restraints, and officers deployed two Tasers. CCTV captured the entire exchange on video but, as Moxley was surrounded by officers, it is not clear from the footage at what point the Tasers were deployed and at what point Moxley went into medical distress.

Although police accounts later suggested Moxley continued to resist officers after he was shocked, a police incident report, written on the day of Moxley’s death and released to the Guardian under a records request, notes he was unresponsive “shortly after the Taser was deployed”.

In an autopsy, also obtained under a records request, pathologist Lisa Kohler acknowledged the video footage did not reveal when the Tasers were deployed and what Moxley’s reactions were. But she noted: “The contribution of the Taser use to this death cannot be confirmed or negated based on the information made available to this pathologist; however the barb injuries are in the area of the heart.”

Nonetheless, the Richland County coroner, who ultimately decided the cause and manner of death, ruled it a drug overdose, describing the manner as accidental. The toxicology reports contained in the autopsy, however, only tested for the presence of synthetic cannabinoid in the blood, not the levels it displayed at.

“We can only put what we can confirm we know,” said Tom Stortz, an investigator in the coroner’s office who worked on the Moxley case. “We can’t put probables or maybes.”

This thinking seems to have been applied in other similar cases. For example, in the death of Brian Pickett II, an autopsy, obtained under a records request, found the 26-year-old African American went into VF shortly after he was shocked in the chest. The Los Angeles County pathologist classed the manner of death as “undetermined” and the cause linked to methamphetamine use. But, in a handwritten note, it is added: “Cannot exclude electromechanical disruption device effects during restraint maneuvers.”

Critics might argue that such a ruling in the Moxley case is a cautious one. From 2005 to 2006, Dr Kohler, Ohio’s first female medical examiner, ruled a homicide in the deaths of three men who, in separate instances, had struggled with police and been Tasered. Use of the ECW was recognised in the cause of each death.

But in an aggressive move, Taser International subsequently sued Kohler and a number of her colleagues under a suit captioned “complaint to correct erroneous cause of death determinations”. A judge ruled for the company and forced Kohler, who has said in interviews that she stands by these determinations, to change the manner of deaths from homicide to either accidental or undetermined and wipe all references to the ECW.

Jeffrey Jentzen, who was president of the National Association of Medical Examiners at the time of Taser’s successful suit, described the legal action as “dangerously close to intimidation”.

In a brief phone interview, Kohler referred all questions on the Moxley case to the county coroner who made the final determination over Moxley’s manner and cause of death.

In other instances, it is alleged that police departments have actively withheld video evidence from pathologists.

Mario Ocasio, a 51-year-old from the Bronx in New York, died on 8 June after a group of NYPD officers hit him with batons and then used a Taser to shock him twice in the back. He was reportedly high on drugs and had been acting violently before officers arrived at his girlfriend’s residence. According to a recently filed federal lawsuit, video of the altercation was captured on the cellphone of Ocasio’s girlfriend, which was subsequently confiscated by police at the local precinct.

Lawyers working for Ocasio’s family contend that this video evidence has not been handed to the pathologist working on the case. Affidavits from eyewitnesses to the altercation state that Ocasio, who had previously served 20 years in prison for the attempted murder of a police officer, was in handcuffs at the time a senior officer deployed the Taser.

“After he was shot [with the Taser],” Ocasio’s girlfriend, Geneice Lloyd, stated in an affidavit, “Mr Ocasio’s last words were, ‘I see God’ he stopped moving or speaking and then he turned blue. He was completely unresponsive and his head movement had completely stopped.”

A spokeswoman for the New York City medical examiner’s office confirmed the cause of Ocasio’s death is still pending, but would not comment on whether video evidence had been reviewed. When contacted by the Guardian, the pathologist involved declined to comment. The NYPD also declined to comment.

A ‘new toy’ for law enforcement, born of sci-fi fantasies

The emergence of electroshock devices in the repertoire of US law enforcement traces back to the civil rights era across the American south and winds between law enforcement, racial tensions and business savvy. In 1963, the New York Times reported that “electric prods” were being used to “herd negro demonstrators” in Alabama. These devices, more commonly known as cattle prods, were designed for and until that point had only been used to herd livestock.

George Bartell, who ran a leading manufacturer of the prods called Hot-Shot Products, “expressed distress” that the devices were being used against people. “We never manufactured them as a law enforcement device,” Bartell said in the article. Within a year, however, his company had patented an electrified police baton.

But those devices never became popular, due in large part to the images of such of devices being aggressively deployed against nonviolent demonstrators.

“Their use,” said Darius Rejali, a political scientist professor at Reed College and an expert in electro-torture, “marked a key moment, when civil rights protesters could say not only that the police were putting them down, but that they were also being treated as animals. That raised a whole series of issues.”

Nearly a decade later, inventor John Cover filed to patent the very first Taser ECW in 1973. Trained as a nuclear physicist, Cover developed the device partly in response to the emerging trend of airplane hijackings. The original Taser was imagined as a gadget that could subdue a prospective hijacker without subjecting the plane to the undue risk of a missed shot from a marshal’s firearm.

Cover drew at least some of his inspiration for the device from a science-fiction series about a young boy named Tom Swift, whose futuristic inventions led him on grand adventures. In one installment, Swift develops an electric rifle and takes it with him to Africa to poach elephants for ivory, and against the native Africans described as “wild, savage and ferocious … like little red apes”.

“Mercifully, Tom and the others ï¬red only to disable and not to kill,” the book reads. “Tom’s electric riï¬‚e was well adapted for this work, as he could regulate the charge to merely stun.”

Cover’s real-world Taser would derive its name from a loose acronym of the book’s title: Tom Swift and His Electric Rifle. Unlike prods, the Taser’s launchable probes, propelled by gunpowder, gave it a range of approximately 15ft, well beyond the close contact needed to deploy a baton shock. Rather than a constant current of electricity over a small patch of skin, the first-generation Taser applied it to a larger area, and in order to cut down on battery size it used short oscillating bursts of current just millionths of a second long.

The Taser’s inventor struggled for decades to discover a market for his device, finding law enforcement agencies more troubled at the prospect of citizens armed with Tasers that could incapacitate officers than they were interested in adding a less lethal device to their arsenals.

But in 1982, the Los Angeles city council banned police from using chokeholds, and civilians did not fare better with the metal batons that the department adopted instead. Excessive force complaints doubled between 1983 and 1988, and the department was, by the end of the decade, recording nearly 900 baton deployments each year.

And so Cover’s “police special” model was born. The PS-83 Taser was retooled from its consumer-focused design into a more rugged, street-ready device. He built the trigger for the grip of a police officer – more powerful and quicker than that of a flight attendant. City officials in Los Angeles placed their first order for 700 devices in 1983.

But as Taser use proliferated among Los Angeles-area police, so did civil liability lawsuits, personnel complaints and disability compensation claims. Officers were often using them without discretion, most infamously in the case of Rodney King, who was beaten by a group of LAPD officers and Tasered three times. In the aftermath of the King incident, Tasers fell into disuse.

In 1993, brothers Rick and Tom Smith formed a small company, Air Taser, using money borrowed from their parents to reinvent Cover’s technology. The first Air Taser device used compressed gas, rather than gunpowder, to propel the electrode barbs, skirting many of the regulatory issues that left Cover’s early models, at various times, to be classified as a firearm.

The Smith brothers renamed their company Taser International in 1998 and launched Project Stealth to develop a more effective weapon that could stop subjects regardless of their mental focus or pain tolerance, culminating in the 1999 release of the M26 Taser.

The new weapon was much more powerful, reimagined by Taser’s designers from a simple long-distance shock weapon into an electromuscular disruption (EMD) device, which emitted electrical signals to effectively override those that the human body uses to trigger muscle movement.

The device impressed law enforcement officials across the country, and sales took off: between 1999 and 2009 the growth rate of ECW penetration in the market was 84%. (The rate of growth for mobile phones over the same time period was slightly more than 5%.)

While EMD technology was adopted by other manufacturers, nearly all of that growth funnelled directly to one firm: in 2002, its first full year as a public company, Taser International posted net sales of just under $7m. The company sold $91m worth of its professional-grade ECW devices in 2014. On Tuesday, Taser posted record quarterly profits of $50.4m, largely on the back of the company’s focus on the new, lucrative market for body cameras and video storage.

The rapid adoption of Tasers offered researchers a novel way to step back and look at the risks they might pose. In 2009, Dr Tseng and colleagues published an epidemiological study on the in-custody death rates of 50 California police departments in the five years before and five years after they adopted ECWs. The study found a startling 600% increase in sudden-death incidents in the year after Taser introduction, and then a 40% increase over pre-Taser rates for the next four years.

Tseng said the rise seemed to indicate that the weapons carry a distinct sudden-death risk, in part because of how fervently Taser International defends its products’ safety.

“You have this new toy and you’re told by Taser International that it’s non-lethal: ‘The world is great because you don’t need to shoot somebody, you don’t need to be close to them to use pepper spray, so have at it,’” Tseng said. “Then they’re recognizing, hey, it’s not non-lethal, it is associated with certain sudden-death risk, we better implement some corrective strategies so we don’t cause excess harm.”

This is what Tseng believes accounts for the decline in sudden death after a department’s first year of Taser implementation.

“Getting hit with a Taser is not pleasant but an analogy I use sometimes is chemotherapy,” Rick Smith said in 2011 interview describing his views on the product’s benefit to society. “If you’ve got cancer, they do awful things to your body to try and save you. Well, our society has a cancer. We are a violent, dangerous society, and we have a device that, while it’s not pleasant, it can make a huge difference and save tons and tons of lives.”

‘Excited delirium’: real science or a cop-out?

You promised that you wouldn’t kill me,” Natasha McKenna whimpered as five Fairfax County deputies advanced outside her Virginia jail cell door, crouched behind a riot shield. “I didn’t do anything.”

McKenna, a diminutive African American woman with a long history of schizophrenia, stood naked and unarmed. She had flung feces and urine in her cell shortly before. The deputies, wearing gas masks and white Hazmat suits, piled in and attempted to place her in restraints before a cell transfer. McKenna was already handcuffed.

The entire “extraction” on 3 February was recorded on video. It unfolded like a horror movie.

McKenna is shoved against the wall with the riot shield and slides to the ground. The struggle continues for about 15 minutes until the 37-year-old is lifted, her head now covered in a black “spit sock” and her hands handcuffed behind her back, onto a restraint chair.

As she moans, she is warned that continued resistance will result in use of the Taser. She is then “drive-stunned”, meaning the weapon is thrust into the skin without deploying the probes, and then Tasered with a full deploy of the probes into her leg three times – each for the full five-second cycle – as officers accuse her of continued resistance. She groans in pain at various points.

After the last shock is administered, McKenna continues to grunt and shake her legs. But slowly her movements begin to fade. Roughly three minutes after the final Taser shock, it appears she is no longer moving.

“If that’s not torture, I don’t know what is,” Harvey Volzer, an attorney for the McKenna family, said in a telephone interview. “When you have someone who is already restrained – her hands handcuffed behind her back, her feet shackled, a spit hood over her head – there’s nothing she could do. Why do you need to Taser someone four times? The answer is: you don’t.”

The videographer, a Fairfax County lieutenant, follows the officers as McKenna’s limp body is wheeled on a restraint chair through the back channels of the Fairfax adult detention center. She is parked by an exit as deputies, still dressed in gas masks, discuss the best way to load her into a transfer vehicle. Her head, still inside the “spit sock”, is slumped forward as a deputy pulls off the blanket, revealing part of her naked body to show the restraints to another officer. Minutes later, after a nurse checks her for a pulse, it is finally realised that McKenna is unresponsive.

“Hey, we’ve got bad news,” a deputy can be heard saying, his voice still muffled inside a gas mask.

McKenna was admitted to hospital in cardiac arrest with lactic acidosis and hyperthermia, according to the detailed account of events provided by the commonwealth’s attorney’s investigation. Despite being revived, after an hour of CPR, she was declared brain-dead. Days later, the life-support machine was switched off.

The medical examiner classified McKenna’s death as accidental, describing the cause as “excited delirium with physical restraint including use of conducted energy device”.

In September, Virginia commonwealth attorney Raymond Morrogh announced that none of the six officers who restrained McKenna would face any criminal charges. He appeared to use this diagnosis to blame McKenna’s own resistance for her death.

“In the end,” wrote Morrogh, “it was McKenna’s severe mental illness, coupled with tremendous physical exertion she put forth over an extended period of time struggling with deputies that resulted in a cascade of lethal chemical reactions inside of her body.”

Volzer, the McKenna family’s attorney, was disgusted. “Excited delirium is junk science,” he said in phone interview. “Why waste all that time if they were going to base it on excited delirium which is a non-reason? It was a joke.”

The term has featured in five of the 19 pathologists’ rulings in the 47 deaths following Taser use so far in 2015. Research by Amnesty International indicates it has been cited as the cause of death in 75 cases after Taser use between 2001 and 2008.

Yet the terminology of “excited delirium”, which loosely refers to a combination of psychotic behaviour, high body temperature and extreme aggression, is not recognised by the American Medical Association or the American Psychological Association. Tuttle, of Taser International, said in a 2007 interview that the company distributed sympathetic literature on the condition, which is recognised by the National Association of Medical Examiners, to pathologists around the United States. He told the Guardian that Taser no longer distributes this information.

“It seems like it is a catch-all,” said Justin Mazzola, a researcher at Amnesty International US who has monitored deaths following Taser use. “It’s not that it’s necessarily letting the officers off the hook, rather than explaining away the negative effects of the Taser.”

Volzer said the McKenna family are planning to file a civil suit. In April, the Fairfax County Sheriff’s Office banned all use of Tasers at the county jail, citing “an unusual event”.

Additional reporting by Jon Swaine and the Guardian US interactive team