Nearly a thousand times this year, an American police officer has shot and killed a civilian.

When the people hired to protect their communities end up killing someone, they can be called heroes or criminals - a judgment that has never come more quickly or searingly than in this era of viral video, body cameras and dash cams. A single bullet fired at the adrenaline-charged apex of a chase can end a life, wreck a career, spark a riot, spike racial tensions and alter the politics of the nation.

In a year-long study, The Washington Post found that the kind of incidents that have ignited protests in many U.S. communities - most often, white police officers killing unarmed black men - represent less than 4 percent of fatal police shootings. Meanwhile, The Post found that the great majority of people who died at the hands of the police fit at least one of three categories: they were wielding weapons, they were suicidal or mentally troubled, or they ran when officers told them to halt.

The Post sought to compile a record of every fatal police shooting in the nation in 2015, something no government agency had done. The project began after a police officer shot and killed Michael Brown in Ferguson, Missouri, in August 2014, provoking several nights of fiery riots, weeks of protests and a national reckoning with the nexus of race, crime and police use of force.

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Race remains the most volatile flash point in any accounting of police shootings. Although black men make up only 6 percent of the U.S. population, they account for 40 percent of the unarmed men shot to death by police this year, The Post's database shows. In the majority of cases in which police shot and killed a person who had attacked someone with a weapon or brandished a gun, the person who was shot was white. But a hugely disproportionate number - 3 in 5 - of those killed after exhibiting less threatening behavior were black or Hispanic.

Regardless of race, in more than a quarter of cases, the fatal encounter involved officers pursuing someone on foot or by car - making chases one of the most common scenarios in the data. Some police chiefs and training experts say more restrictive rules on when to give chase could prevent unnecessary shootings.

Like a growing number of police shootings, the death of David Kassick on a snow-covered field near his sister's house in Hummelstown, Pennsylvania, was captured on video - a technological shift that has dramatically altered how Americans perceive officers' use of deadly force.

In two minutes and 10 seconds of harrowing footage, the Kassick video serves as an almost perfect Rorschach test in the national debate over when it is justifiable for an officer to take a life.

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'Shots fired'

Officer Lisa Mearkle has chased Kassick, first by car, then on foot. Now she's zapped him with her Taser and he's writhing on the ground, on snow, jammed up against a line of trees.

Viewed through the camera attached to the officer's Taser, Kassick reacts to each of three shocks from the stun gun. Mearkle, screaming, orders Kassick, who is already involuntarily on the ground, to "Get on the ground! Get on the ground!"

"Okay, okay," he responds.

As the officer stands over Kassick, repeatedly ordering him to "Lie down" and "Show your hands," the 59-year-old does just that. He moans in pain, pulls his right hand out from under his head and stretches to display the hand.

But three times during the video, Kassick also does other things with his hands. As he says "Okay, okay" to the officer's command, he also reaches toward his jacket pocket. A little later, his left hand moves toward his front pants pocket. He appears to be trying to remove Taser wires from his clothing. Thirty seconds later, he uses his left hand to lift himself slightly from the snow.

At the 1:39 mark, there's a pop and Mearkle says, "Shots fired."

Within seconds, Kassick is flat on his stomach. He lifts his head. The officer, calm now, says, "Keep your hands where I can see them."

The video ends. Kassick is dead, shot twice in the back.

He was unarmed.

Mearkle had given chase after Kassick fled from her attempt to pull him over for having an expired inspection sticker on his car.

Video: A Pennsylvania jury recently acquitted police officer Lisa Mearkle in the fatal shooting of 59-year-old David Kassick last February. The unarmed man was Tasered then shot twice while fleeing an attempted traffic stop for expired inspection and emission stickers. According to Washington Post data, in more than a quarter of cases where an officer fatally shot a civilian this year, that encounter involved a vehicle or foot pursuit. (Jorge Ribas / The Washington Post)

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The video age

In today's tinderbox of public concern about police brutality, video of shootings can be damning evidence or a clear defense. Police chiefs and politicians like video because in most cases it absolves officers of allegations of wrongdoing. Civilians like video because when officers do act abusively, digital proof makes coverups unlikely.

In the Kassick case, some of Mearkle's defenders argue that intricate inspection of videos warps perceptions of the challenges police face. A system in which officers make split-second decisions - but in which their bosses, prosecutors, jurors and the public have the luxury of examining every frame of video - is unfair, said Les Neri, president of the Pennsylvania Fraternal Order of Police.

"We now microscopically evaluate for days and weeks what they only had a few seconds to act on," Neri said. "People always say, 'They shot an unarmed man,' but we know that only after the fact. We are criminalizing judgment errors."

The decisions police officers must make in a flash can have fatal consequences - for themselves as well as for suspects. Thirty-six officers have been shot and killed in the line of duty this year, according to the Officer Down Memorial Page.

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The widespread availability of video of police shootings - from bystanders' smartphones as well as from police body and dashboard cameras - has been a primary factor in the rising number of indictments of officers.

Prosecutors cited video evidence against officers in 10 of the 18 felony cases filed against officers this year - twice as often as video played a role in prosecutions over the previous decade, The Post found.

"Thank God for technology," said the Rev. Ira Acree, pastor at Greater St. John Bible Church in Chicago, where Officer Jason Van Dyke faces a first-degree murder charge for shooting 16 rounds and killing Laquan McDonald, a 17-year-old who was walking down the middle of the street holding a three-inch knife. "Maybe it's finally helping us crack the blue code of silence."

After police dash-cam video of the 2014 incident was released last month, Mayor Rahm Emanuel (D) fired the city's police chief.

"In the past, an officer's word was not challenged," said Philip M. Stinson, a criminologist at Bowling Green State University who studies arrests of officers. "If anything has shifted this year, it's that. They are facing the kind of scrutiny the rest of us face when we kill someone."

But some officers' friends and attorneys attribute the uptick in prosecutions to rising political pressure. On a fundraising website, supporters of West Monroe, Louisiana, officer Jody Ledoux blamed his January felony negligent-homicide indictment on "our country's current climate towards police." Ledoux's attorney, Mickey DuBos, did not return calls seeking comment.

Ledoux killed Raymond Martinez, a homeless 51-year-old, the day after a grand jury in New York City declined to bring criminal charges against Officer Daniel Pantaleo, who was recorded last year putting a fatal chokehold on Eric Garner, a Staten Island man who was stopped for selling loose cigarettes. The decision not to charge Pantaleo sparked nationwide protests.

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Surveillance video in the Louisiana case shows Ledoux shot Martinez as he reached into a newspaper vending machine in front of a convenience store to retrieve his cellphone. Ledoux said he feared Martinez was reaching for a gun.

Although more officers were indicted in shooting cases this year, the outcome of such cases improved for officers. Five of the seven cases tried this year ended with the officer acquitted or with a mistrial. In two cases, charges were dismissed. Over the previous decade, one-third of officers charged in shooting cases were convicted of crimes ranging from misdemeanor reckless discharge of a firearm to felony murder.

This year, only one officer, Richard Combs, former chief of a small department in Eutawville, South Carolina, pleaded guilty. In September, following two mistrials on a murder charge, he pleaded to a misdemeanor charge of misconduct in office and was sentenced to one year of home detention after he fatally shot Bernard Bailey in a parking lot. Bailey had resisted arrest on a warrant in 2011.

As protests have increased pressure for transparency about fatal shootings, more departments have moved to equip officers with body cameras. Many chiefs say the cameras boost public confidence in the police, but most departments do not yet use them. About 6 percent of fatal shootings this year were captured by body cameras, according to The Post's database.

Where cameras are used, police often refuse to publicly release video. In more than half the cases in which body-cam footage was available, police declined The Post's requests to make the video public. Officials said releasing footage before cases are closed could taint jury pools, making it difficult to win convictions.

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Judging 'mindset'

Officer Mearkle killed Kassick in February and was charged with third-degree murder, manslaughter and involuntary manslaughter. Eight months later, 12 jurors sat in judgment. Mearkle, 37, faced up to 40 years in prison and the end of her career.

Mearkle, who would later express regret for Kassick's death, testified that she had "no doubt" that Kassick - who was a heroin addict, though the officer didn't know that when she gave chase - was reaching for a weapon when he moved toward his jacket pocket as he squirmed in the snow. "There was no reason for him to reach into his frigging pocket!" she yelled in court.

She could not let Kassick escape, she said, because someone who runs from an officer might be a danger to the community. "Something is wrong here," she testified, recalling her thinking at the start of the chase. "This is not normal for someone to flee the police."

Last month, after 11 hours of deliberations, the jury acquitted Mearkle of all charges.

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The jury foreman, Scot Benoit, says he would not have shot Kassick. After watching the video eight times, Benoit and some of his fellow jurors concluded it was not necessary to shoot the man on the ground. But that is not the question they were asked to consider.

"Our job was to look at her mind-set," Benoit said. "We had to determine if her fears were justified."

To figure that out, the jury had to look beyond the video. One fact weighed heavily on jurors: When the chase started, Kassick, trying to pull away from Mearkle, steered around another vehicle that was stopped at a red light.

"That escalated the situation in Officer Mearkle's mind," Benoit said. "Quite clearly, he was eluding the police and she didn't know why. The prosecutor kept saying this was just over an inspection sticker. But when Kassick went around the other vehicle, he's fleeing at a high rate of speed on a residential street and kids are coming home from school, so I could see where she's coming from."

Kassick's sister, Diane Fetters, says it was her brother who had reason to be afraid, not the officer. "He just panicked," she said. "He was afraid of going to jail because he was driving without a license. Her adrenaline kicked in and she wasn't able to deal with it. She had plenty of opportunity to back off.

"I mean, what she was pursuing him for, the expired sticker? She could have just sent him a summons in the mail."

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A lack of data

The landscape of police shootings is surprisingly thinly explored. The FBI is charged with keeping statistics on such shootings, but a Post analysis of FBI data showed that fewer than half of the nation's 18,000 police departments report their incidents to the agency.

The Post documented well more than twice as many fatal shootings this year as the average annual tally reported by the FBI over the past decade. The FBI and the federal Bureau of Justice Statistics now acknowledge that their data collection has been deeply flawed. FBI Director James B. Comey called his agency's database "unacceptable." Both agencies have launched efforts to create new systems for documenting fatalities.

The FBI will replace its current program with a "near real-time" database to be made public by 2017, said Stephen L. Morris, a senior FBI official whose division is responsible for collecting crime data.

"We are responding to a real human outcry," Morris said. "People want to know what police are doing, and they want to know why they are using force. It always fell to the bottom before. It is now the highest priority."

The Post's database, compiled from interviews, police reports, local news accounts and other sources, tracked more than a dozen details about each killing, including the events that led to the fatal encounter, whether the slain person was armed, and demographic data on each person. The Post will continue tracking fatal shootings by police in 2016.

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The research also noted whether victims were mentally ill or experiencing an emotional crisis, a category that came to account for one-quarter of those killed. Officers fatally shot at least 243 people with mental health problems: 75 who were explicitly suicidal and 168 for whom police or family members confirmed a history of mental illness.

The analysis found that about 9 in 10 of the mentally troubled people were armed, usually with guns but also with knives or other sharp objects. But the analysis also found that most of them died at the hands of police officers who had not been trained to deal with the mentally ill.

"Often they have an edged weapon, like a knife, and when officers start yelling, 'Drop it! Drop it!' that will not calm them down," said Chuck Wexler, executive director of the Police Executive Research Forum, a Washington police think tank. "Instead, it increases their anxiety."

In most of those cases, police were called by a relative or a neighbor who was worried about a mentally fragile person's erratic behavior. Yvonne Mote of Alabama dialed 911 in March out of desperation, hoping police could help her brother, Shane Watkins, who suffered from schizophrenia. Instead, he wound up dead.

"A week after they killed my brother, there was an armed robbery," Mote said. "That guy had a gun, and they arrested him without killing him. Why did they have to kill my brother, who only had a box cutter? I still don't understand."

The prosecutor in the Mearkle case, Johnny Baer, still says it was right to charge the officer with murder. She was "out of control," he said.

In court, Baer told jurors that "anytime anyone involved in an encounter with a police officer doesn't show their hands, that isn't a reason to shoot. Ninety-nine point nine percent of police officers use extraordinary restraint in these situations."

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But, Baer said weeks after the trial, "we had a conservative central-Pennsylvania jury and a female officer who is a mother and who was tearful and emotional in court."

It remains unusual for police to face criminal charges in fatal shootings, but the indictment rate in such cases more than tripled this year - a striking shift in the willingness of prosecutors to charge officers.

The Post found that an average of five officers per year have been indicted on felony charges over the previous decade; this year, 18 officers have been charged with felonies including murder, manslaughter and reckless discharge of a firearm.

Such accusations rarely stick, however. Only 11 of the 65 officers charged in fatal shootings over the past decade were convicted.

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Guiding principles

Aftershocks of the Mearkle case still reverberate in Hummelstown and beyond. Several thousand people signed an online petition asking that the town not reinstate Mearkle to her police job. No decision has been made on that.

Mearkle, whose criminal and civil attorneys did not return repeated calls from The Post, said at a news conference after the verdict that she is determined to return to her job and is sorry about the shooting.

"I truly wish it didn't happen, and I want [the family] to know that I never wanted to shoot anybody," she said.

Police departments design rules and training with the aim of resolving difficult situations without shooting anyone. But the rules vary enormously. About half of departments allow officers to give chase no matter what offense a suspect has committed, while the other half limits pursuits to certain kinds of offenses, according to a study by the International Association of Chiefs of Police.

The Post's database shows that nearly 1 in 3 shootings that result from a car chase start with a traffic stop for a minor infraction.

In recent years, pursuit policies have generally grown tighter. Old rules that left the decision to "officer judgment" have been replaced by sometimes complex matrixes requiring police to weigh the severity of the crime being committed before they decide whether to give chase.

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After Las Vegas police in 2009 adopted a use-of-force policy requiring officers to put the highest premium on "the sanctity of human life," some other departments followed suit. Four years after the change in Las Vegas, the city's officer-involved shootings had fallen by nearly half.

"That is a real sign of the times, a new kind of language that changes police culture," said Wexler, whose organization recommends tighter pursuit policies. "The guiding principle has to be proportionality: Is my action proportional to the act being committed? We've recommended that the policy has to be ironclad, because if you say 'except if the officer fears for his life,' inevitably they will say they fear for their life."

When New York, Boston and other big cities tightened rules on pursuits, they saw a sharp decline in the number of officers who shot at vehicles.

"Good cops judge when they can hold back," said Geoffrey Alpert, a criminologist at the University of South Carolina who has studied pursuits for three decades. "So what if you get pushed in a volatile domestic situation? You're justified to use force, but you tactically withdraw, calm them down and move on."

More-restrictive pursuit policies are no panacea, however. Although many experts support the change, a review by George Mason University criminologist Cynthia Lum of 33 studies of pursuit policies concluded that tightening the rules led to fewer police injuries - but also more crime.

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Still, Rob Ord, a longtime instructor on defensive police tactics who now runs Falken Industries, a Virginia security company, said, "It's almost always better to back off and call for help."

When Ord was a police officer in Florida, he was directing traffic one day when a driver disobeyed his command to turn left. Instead, the driver gunned his engine and drove straight at Ord.

"I rolled onto his hood, firearm drawn," Ord recalled. "My finger was on the trigger, ready to pull."

"And I stopped. I did not fire," he said. "That person's alive and he was charged, and I'm alive and I have a house and a job and I wasn't sued. I'm happy."

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Steven Rich and Kimbriell Kelly contributed to this report.