In 2013, Fort Worth officers served a no-knock warrant in search of cocaine. Jermaine Darden, 34, who was obese and had asthma, was handled roughly and stunned with a Taser. When medics arrived, he was unresponsive. The medical examiner ruled he died of cardiac-related natural causes.

CORNELIA, Ga. — This town on the edge of the Appalachians has fewer than 5,000 residents, but the SWAT team was outfitted for war.

At 2:15 a.m. on a moonless night in May 2014, 10 officers rolled up a driveway in an armored Humvee, three of them poised to leap off the running boards. They carried Colt submachine guns, light-mounted AR-15 rifles and Glock .40-caliber sidearms. Many wore green body armor and Kevlar helmets. They had a door-breaching shotgun, a battering ram, sledgehammers, Halligan bars for smashing windows, a ballistic shield and a potent flash-bang grenade.

The target was a single-story ranch-style house about 50 yards off Lakeview Heights Circle. Not even four hours earlier, three informants had bought $50 worth of methamphetamine in the front yard. That was enough to persuade the county’s chief magistrate to approve a no-knock search warrant authorizing the SWAT operators to storm the house without warning.

The point man on the entry team found the side door locked, and nodded to Deputy Jason Stribling, who took two swings with the metal battering ram. As the door splintered near the deadbolt, he yelled, “Sheriff’s department, search warrant!” Another deputy, Charles Long, had already pulled the pin on the flash-bang. He placed his left hand on Deputy Stribling’s back for stability, peered quickly into the dark and tossed the armed explosive about three feet inside the door.

It landed in a portable playpen.

As policing has militarized to fight a faltering war on drugs, few tactics have proved as dangerous as the use of forcible-entry raids to serve narcotics search warrants, which regularly introduce staggering levels of violence into missions that might be accomplished through patient stakeouts or simple knocks at the door.

Thousands of times a year, these “dynamic entry” raids exploit the element of surprise to effect seizures and arrests of neighborhood drug dealers. But they have also led time and again to avoidable deaths, gruesome injuries, demolished property, enduring trauma, blackened reputations and multimillion-dollar legal settlements at taxpayer expense, an investigation by The New York Times found.

For the most part, governments at all levels have chosen not to quantify the toll by requiring reporting on SWAT operations. But The Times’s investigation, which relied on dozens of open-record requests and thousands of pages from police and court files, found that at least 81 civilians and 13 law enforcement officers died in such raids from 2010 through 2016. Scores of others were maimed or wounded.

The casualties have occurred in the execution of no-knock warrants, which give the police prior judicial authority to force entry without notice, as well as warrants that require the police to knock and announce themselves before breaking down doors. Often, there is little difference.

Show of Force

Innocents have died in attacks on wrong addresses, including a 7-year-old girl in Detroit, and collaterally as the police pursued other residents, among them a 68-year-old grandfather in Framingham, Mass. Stray bullets have whizzed through neighboring homes, and in dozens of instances the victims of police gunfire have included the family dog.

Search warrant raids account for a small share of the nearly 1,000 fatalities each year in officer-involved shootings. But what distinguishes them from other risky interactions between the police and citizens, like domestic disputes, hostage-takings and confrontations with mentally ill people, is that they are initiated by law enforcement.

In a country where four in 10 adults have guns in their homes, the raids incite predictable collisions between forces that hurtle toward each other like speeding cars in a passing lane — officers with a license to invade private homes and residents convinced of their right to self-defense.

After being awakened by the shattering of doors and the detonation of stun grenades, bleary suspects reach for nearby weapons — at times realizing it is the police, at others mistaking them for intruders — and the shooting begins. In some cases, victims like Todd Blair, a Utah man who grabbed a golf club on the way out of his bedroom, have been slain by officers who perceived a greater threat than existed.

To be sure, police officers and judges must find probable cause of criminal activity to justify a search warrant. Absent resources for endless stakeouts, police tacticians argue that dynamic entry provides the safest means to clear out heavily fortified drug houses and to catch suspects with the contraband needed for felony prosecutions.

But critics of the forcible-entry raids question whether the benefits outweigh the risks. The drug crimes used to justify so many raids, they point out, are not capital offenses. And even if they were, that would not rationalize the killing or wounding of suspects without due process. Nor would it forgive the propensity of the police to err in the planning or execution of raids that are inherently chaotic and place bystanders in harm’s way.

Forcible-entry methods have become common practice over the last quarter century through a confluence of the war on drugs, the rise of special weapons and tactics squads, and Supreme Court rulings that have eroded Fourth Amendment protections against unreasonable searches. Support for their continued use has been bolstered by an epidemic of opioid abuse and the threat of domestic terrorism.

Because many raids occur in low-income neighborhoods, shooting deaths like one in November of a 22-year-old black man in Salisbury, N.C., have exacerbated racial tensions already raw from a spate of high-profile police killings. The American Civil Liberties Union concluded in a recent study of 20 cities that 42 percent of those subjected to SWAT search warrant raids were black and 12 percent Hispanic. Of the 81 civilian deaths tallied by The Times, half were members of minority groups.

About the Data A full list of fatalities resulting from forcible-entry search warrant raids from 2010 to 2016, tallied by The Times, along with demographic details.

The no-knock process often begins with unreliable informants and cursory investigations that produce affidavits signed by unquestioning low-level judges. It is not uncommon for the searches to yield only misdemeanor-level stashes, or to come up empty.

In some instances when officers have been killed, suspects with no history of violence, found with small quantities of drugs, have wound up facing capital murder charges, and possible death sentences.

In December, a jury in Corpus Christi, Tex., acquitted a 48-year-old man who spent 664 days in jail after being charged with attempted capital murder for wounding three SWAT officers during a no-knock raid that targeted his nephew. The jury concluded that the man, Ray Rosas, did not know whom he was firing at through a blinded window.

While the officers are typically seeking narcotics, there also have been deaths and serious injuries when warrants were served on people suspected of running illegal poker games, brewing moonshine and neglecting pets. In 2011, officers in Marine City, Mich., conducted a dynamic-entry raid to serve a search warrant for “any and all evidence pertaining to graffiti including but not limited to, spray paint containers, markers, notebooks, and photographs.” After forcing residents to the floor at gunpoint, they found nothing, according to depositions by the residents.

The Times found that from 2010 to 2015, an average of least 30 federal civil rights lawsuits were filed a year to protest residential search warrants executed with dynamic entries. Many of the complaints depict terrifying scenes in which children, elderly residents and people with disabilities are manhandled at gunpoint, unclothed adults are rousted from bed and houses are ransacked without recompense or apology. Louise Milan, 68, of Evansville, Ind., alleged in her filing that she and her 18-year-old daughter were handcuffed in front of neighbors during a door-busting 2012 raid prompted by threats against the police made by someone who had pirated her wireless connection.

“There’s a real misimpression by the public that aggressive police actions are only used against hardened criminals,” said Cary J. Hansel, a Baltimore lawyer who has represented plaintiffs in such lawsuits. “But there are dozens and dozens of cases where a no-knock warrant is used against somebody who’s totally innocent.”

At least seven of the federal lawsuits have been settled for more than $1 million in the last five years. They include a $3.75 million payment in 2016 to the family of Eurie Stamps, the unarmed Framingham grandfather who was accidentally shot while compliant and on his stomach; and $3.4 million in 2013 to the family of Jose Guereña, a 26-year-old former Marine shot more than 20 times as agents broke into his house in Tucson. No drugs were found.

In each of those cases, as in almost all botched raids, prosecutors declined to press charges against the officers involved.

Planned in Haste

Perhaps no fiasco illustrates the perils of no-knock searches as graphically as the 2014 raid here in Georgia’s northeast corner. On May 22, an eager young Habersham County sheriff’s deputy named Nikki Autry, who was attached to a narcotics task force, turned a small-time methamphetamine user into a confidential informant. Intent on avoiding jail, the informant, James Alton Fry Jr., set about the task of baiting bigger fish.

According to trial testimony and investigative documents, the agents sent Mr. Fry out on the night of May 27 to make drug buys. He scored two Lortab pain pills on his first approach, struck out with a second source and then was connected to a meth dealer named Wanis Thonetheva. At around 10:30 p.m., Mr. Fry, his wife, Devon, and their housemate, Larry Wood — all persistent meth users — drove to the address provided by the dealer.

“It didn’t look like a drug house,” Ms. Fry later testified. “This was a nice house. It’s usually a shack or trailer.” The police did not follow them to provide protection or surveil the property.

Mr. Wood conducted his business out front with Mr. Thonetheva, a 30-year-old American-born son of Laotian immigrants, as the Frys waited in their red pickup. All three appeared shaken when they met up with their handlers in a church parking lot. They had spotted two men at the house whom they took to be guards for the drug operation, and a third who might have been a supplier.

The agents sent the informants home, but about half an hour later Deputy Autry texted Ms. Fry with an afterthought. “Did y’all see any signs of kids at wanis’ house,” she asked.

“Nothing except a mini van,” came the response.

Thinking she was on to a big score, Deputy Autry, who was 28, did not wait for daylight or further investigation. She returned to the Sheriff’s Office, where she pulled Mr. Thonetheva’s criminal history and mug shot. With the approval of the sheriff, Joey Terrell, she alerted the county’s Special Response Team to prepare for a raid. She and her drug unit commander, Murray J. Kogod, began drafting the application for the no-knock warrant.

The affidavit included inaccuracies and hyperbole. It asserted incorrectly that Mr. Fry — the only informant formally certified by the police — had bought the drugs, rather than Mr. Wood. Deputy Autry described Mr. Fry as “a true and reliable informant,” even though he had not made a buy before that night. Despite the lack of surveillance, she wrote that she had “confirmed that there is heavy traffic in and out of the residence.”

Shortly after midnight, Deputy Autry and another agent awakened the county magistrate, James N. Butterworth, with a house call. He read the affidavit and placed her under oath.

She told him that Mr. Thonetheva had been arrested several times for drug possession, that there might be armed lookouts at the house and that an assault involving an AK-47 had been reported there the previous year. The judge, who had never denied Deputy Autry a warrant, found no reason to dispute probable cause and signed at 12:15 a.m.

“If you had drugs and you had weapons, that was constitutional purpose to go on in there, not to knock on the door,” he later testified.

The Special Response Team, formed three years earlier, consisted of a dozen men plucked from the Sheriff’s Office and the Cornelia Police Department. They trained on their own time for four hours each Thursday. The Humvee had been procured through a Pentagon program that made surplus military equipment available to even the most rural departments.

There had been few chances to quell riots and subdue active shooters in the hamlets of Habersham County, population 43,000. Instead, the unit had been used primarily to serve narcotics search warrants, 32 times in all.

During their pre-raid briefing, team members circulated a photograph of Mr. Thonetheva, a Google Earth image of the brick house with dark shutters and a sketch of the three-bedroom interior. Deputy Autry mistakenly told the team’s commander that the drug deal had gone down near a side door to an enclosed garage, so he plotted his entry from there. She told him there were no signs of children or animals, failing to mention the minivan.

When the flash-bang detonated with a concussive boom, a blinding white light filled the room. The entry team rumbled in, screaming for the occupants to get to the ground. Deputy Stribling peered into the playpen with a flashlight and found 19-month-old Bounkham Phonesavanh.

Deputy Stribling waved off Deputy Long, who had lobbed the grenade. “Charlie, go away, you don’t need to see this,” he said.

The child, known affectionately as Bou Bou, had a long laceration and burns across his chest, exposing his ribs, and another gash between his upper lip and nose. His round, cherubic face was bloodied and blistered, spackled with shrapnel and soot. The heat had singed away much of his pillow and dissolved the mesh side of the playpen.

At first the child was silent. But as Deputy Stribling picked him up, rubbed his feet and shook his arms, he began to wail. Even the drug agents stationed outside the other end of the house could hear the screams.

“You don’t think that baby got hurt, do you?” one asked another.

‘A Secret World’

SWAT was pioneered in Los Angeles and other big cities in the late 1960s and early 1970s in response to civil unrest and raging firefights. Today, almost every police agency with at least 100 officers, and about a third of all smaller ones, either has its own full-time unit or participates in a part-time or multijurisdictional team, according to the Bureau of Justice Statistics.

Once the teams were formed, their existence had to be justified. Drug searches became the answer. Dr. Peter B. Kraska, a criminologist at Eastern Kentucky University, estimates that SWAT deployments increased roughly fifteenfold between 1980 and 2000 as the war on drugs escalated.

There is no way to know the true number of forcible entries because there is no federal mandate that police agencies report on SWAT operations. Only two states have required it, and efforts by news and watchdog groups to compile national figures have been frustrated by police stonewalling.

“This is kind of a secret world within a secret world,” said Dr. Tom Nolan, a criminologist at Merrimack College and a SWAT veteran of the Boston Police Department. “They don’t open up.”

Utah is currently the only exception. In 2014, after a marijuana raid resulted in the death of one officer and the wounding of five (as well as of the suspect, who hanged himself in jail), the state enacted a law requiring annual disclosures about tactical raids.

The initial results show that many departments use dynamic entry almost by default. SWAT units in Utah did so in 61 percent of 1,016 deployments reported in 2014 and 2015. About 40 percent of the warrants they served were no-knocks, usually for drugs, mostly at night.

Maryland had a similar reporting requirement from 2010 to 2014, inspired by a botched raid that resulted in the shooting of two dogs at the home of the mayor of Berwyn Heights. More than 90 percent of 8,249 SWAT deployments during those years were to serve search warrants, and more than two-thirds involved forcible entry. Firearms were discharged in 99 operations, civilians were killed in nine and injured in 95, officers were injured in at least 30 and animals were killed in 14.

C. Anthony Muse, a state senator who sponsored the measure, said he found the numbers “alarming.” He also found it disturbing that the General Assembly allowed the reporting requirement to sunset, amid heavy lobbying from law-enforcement groups. “They don’t want to be held accountable,” he said.

Among the cities examined in the recent ACLU study was Little Rock, Ark., where the SWAT team broke down doors and detonated flash-bangs in more than 90 percent of 147 narcotics search warrant raids between January 2011 and March 2013, according to data shared with The Times.

Deadly outcomes did not slow them down. In 2010, two team members were shot, neither fatally, and a suspect was killed, in a no-knock raid that uncovered half an ounce each of marijuana and crack. Less than two years later, a SWAT officer killed an armed 31-year-old man during an early morning raid that turned up three marijuana plants. The unit then conducted five more raids in the ensuing two weeks.

The shootings prompted a new policy that the team must withdraw if it encounters an armed suspect, but they did not prompt broader reconsideration.

“It’s not foolproof, not always going to happen just right,” said the SWAT commander, Lt. Tim Calhoun. “But it winds up being the most safe for us. These are dangerous people we’re dealing with.”

Summoning armed criminals outside, he said, is no less risky than startling them inside. Long stakeouts exhaust manpower and can lead to dangerous vehicle pursuits, street confrontations and traffic stops that do not yield as much contraband.

It matters little, Lieutenant Calhoun said, that drug crimes may not always pose an immediate mortal threat. “If you have a dope house next door,” he said, “there’s probably nothing the police can do that would be overreacting.”

Shifting Definitions

The mission creep for the heavily militarized teams has coincided with a stark shift toward law enforcement in the Supreme Court’s search for the proper balance between public safety and individual rights.

The quest for that sweet spot dates from the early 17th century and the “castle doctrine” of English common law, which protected residents from unannounced government intrusions. That ethos was enshrined in the United States Constitution in 1791 with the ratification of the Fourth Amendment, which prohibits “unreasonable searches and seizures” and requires that warrants be backed by probable cause.

More than a century ago, those rights seemed so sacrosanct that the Supreme Court, in its early iterations of the exclusionary rule, asserted that evidence obtained through an improper search could not be used at trial. But after a series of rulings in the 1990s, officers became able to obtain a warrant to forcibly enter a house with merely a “reasonable suspicion” that announcing would be dangerous or allow the destruction of evidence. Paradoxically, that standard allowed the use of the most extreme force in pursuit of the smallest amounts of drugs, since a few grams are more quickly flushed than a few bales.

“If it’s a small quantity, you’re probably not going to find too many police investigators that are going to knock,” Chief William P. McManus of the San Antonio Police Department said in a 2010 deposition.

In 2003, the Supreme Court affirmed the right of officers to break into a residence with a standard warrant after knocking and waiting only 15 to 20 seconds. Three years later, it undercut even that requirement by concluding that evidence remains admissible even when the police barge in more quickly.

That has helped blur the distinction between no-knock and knock-and-announce entries. “Either the door comes in and people are yelling ‘Sheriff’s office’ or people yell ‘Sheriff’s office’ and the door comes in,” said Judge Brian M. Rickman of the Georgia Court of Appeals, who was district attorney in Habersham County during the Cornelia raid.

In The Times’s inventory, 47 civilians and five officers died as a result of the execution of knock-and-announce searches, while 31 civilians and eight officers died in the execution of no-knock warrants. The type of warrant could not be determined in three civilian fatalities.

With so little restraint exerted by the federal courts, few states have offered greater protections. A survey by The Times found that 13 states have enacted laws authorizing no-knock warrants. Another 13 have blessed them through rulings by appellate courts. In seven states, no-knock warrants are routinely granted in the absence of explicit authority by statute or the courts. In 16 states and the District of Columbia, no-knock warrants are not customary but the police can nonetheless make unannounced entries with standard warrants.

Most States Allow No-Knock Searches The legal status of no-knock search warrants differs from state to state. In some, they are authorized by law or judicial precedent. In others, they are granted by judges in the absence of statute or case law when exigent circumstances exist, like a risk of danger or destruction of evidence. Oregon is the only state with a law requiring the police to always announce themselves before serving a search warrant. Ore. Wash. Iowa Del. Mich. Va. Alaska W.Va. Idaho N.M. Ala. Conn. Pa. Fla. Ky. Nev. Hawaii N.C. Okla. Tenn. Wis. Ark. Calif. N.D. Ga. Me. Ill. La. S.D. Minn. Mo. Neb. Kan. Ariz. Utah Colo. N.H. S.C. Ind. Miss. Mont. N.J. N.Y. Md. Mass. Ohio Tex. Wyo. Vt. No-knock warrants are routinely granted Not granted, but forcible entries allowed under exigent circumstances Police must announce themselves OR WA IA MI VA AK WV ID NM AL CT PA FL KY NV HI NC OK TN WI AR CA ND GA ME IL LA SD MN MO NE KS AZ UT CO NH SC IN MS MT NJ NY MD MA OH TX WY VT No-knock warrants are routinely granted Not routinely granted, but forcible entry allowed under exigent circumstances Police must announce

Only in Oregon does state law prohibit no-knock entries, and only in Florida has a state Supreme Court disallowed no-knock warrants. That 1994 opinion recognized a “staggering potential for violence to both occupants and police.” In 2015, the Utah Legislature banned forcible entry if the only suspected offense is drug possession.

Elsewhere, even when prompted by sensational headlines, legislative efforts to curb the raids have encountered resistance. Several proposals withered in Georgia’s General Assembly the year after the Baby Bou Bou episode, including prohibiting no-knock raids between 10 p.m. and 6 a.m.

Those failures only deepened mistrust in minority neighborhoods, said State Senator Vincent Fort, a Democrat who sponsored the bills in the Republican-controlled legislature. “It makes some community members see the police as occupying forces as opposed to community partners against crime,” he said.

Why It Can Go Wrong

And so the raids continue. When successful, they can temporarily disrupt the supply chain. But too often, the show of force is disproportionate, the errors disastrous. There are a variety of contributing factors.

Inadequate Judicial Scrutiny

In August 2015, an assistant clerk-magistrate in Worcester, Mass., reviewed an application for a no-knock warrant aimed at finding two illegal guns in a drug dealer’s apartment. The affidavit by a state trooper relied solely on a single confidential informant. There had been no surveillance, and the affidavit itself acknowledged that motor vehicle and utility records indicated that the suspect did not live there anymore.

Nonetheless, the magistrate, Brenda D. Seaver, signed the warrant and at about 5 a.m. on Aug. 19 the city’s SWAT team broke through the door. The new tenants — a man and woman who were engaged, their male roommate and two children — were detained at gunpoint as their belongings were ransacked. Both men were handcuffed; the woman, who had been sleeping nude, reported being berated profanely in front of her children.

No contraband was found, and the police eventually became convinced that the residents did not know the suspect, who had moved out more than three months earlier.

The tenants have filed a federal lawsuit against the city, the police and the informant. But their lawyer, Héctor E. Piñeiro, also blames the judiciary.

“If you’re a magistrate,” he said, “you should be asking are there children there, what steps have been taken to find out, what kind of surveillance has been done, how reliable is this confidential informant, are there other ways you can arrest this person. It never gets done.”

Ms. Seaver did not respond to a request for comment.

Wrong Addresses

The mistakes might be laughable were they not so consequential.

In May 2010, the police in Hempstead, N.Y., shot and wounded 22-year-old Iyanna Davis during a no-knock raid at a two-family residence where she lived in an upstairs unit with a separate entrance. The warrant was for downstairs.

Ms. Davis was awakened at about 7 a.m. by the sound of a door’s being smashed and hid in a closet, she recounted in a deposition. A Nassau County police officer, armed with an assault rifle, opened the door, found her crouching and screamed at her to raise her hands.

“That’s when I heard the shot,” she recounted. “The force actually knocked me back on my backside.” The bullet had entered her right breast and exited her abdomen.

In his own deposition, the officer, Michael Capobianco, said that he “tripped and didn’t mean to fire.” He was cleared of any policy violations; Ms. Davis, who spent a week in the hospital and another three months recuperating, won $650,000 in a legal settlement from the county.

Failure to Knock

The warrant to search Julian Betton’s apartment did not authorize the police in Myrtle Beach, S.C., to break in without warning, and the officers insist that they knocked and announced.

But video from Mr. Betton’s front porch surveillance camera on the afternoon of April 16, 2015, shows no sign of warning before drug agents rammed through his door. The recording does not include audio, but a neighbor who watched from steps away told state investigators he was “100 percent sure” there was no prior notice.

Suspected of small-scale marijuana dealing, Mr. Betton said in a deposition late last year that he emerged from a bathroom, saw moving shadows, assumed he was being robbed and reached for the handgun in his waistband. Three officers responded by spraying the room with dozens of rounds, hitting Mr. Betton nine times.

“I just seen stuff coming at me and I went to reach for it and went down,” Mr. Betton, who was 30 at the time, said in a deposition for the lawsuit he later filed.

Mr. Betton awoke from a coma six weeks later, partially paralyzed, having lost portions of his gallbladder, colon, bowel and rectum. On Thursday, he pleaded guilty to drug charges, including possession of nearly eight ounces of marijuana found in the raid. Counts of pointing and presenting a weapon were dismissed and he was sentenced to the time he served in the hospital.

Members of the police team initially claimed that Mr. Betton fired first, but the state crime lab found no evidence that his gun had been discharged. A prosecutor cleared the officers, concluding that the shooting was justified.

Lack of Monitoring

Because fewer than a third of tactical teams use body cameras, the police version of events like the killing of Eric John Senegal is often the only one.

Members of the Beauregard Parish sheriff’s special operations team told investigators that when they broke into Mr. Senegal’s house trailer in rural Ragley, La., on Jan. 4, 2016, they found the slightly-built, 27-year-old man just inside the entrance, pointing a handgun their way. It was not cocked, but two deputies fired, hitting Mr. Senegal three times, according to investigative reports. Another killed his charging pit bull.

Several team members told investigators there were shouts of “Sheriff’s Office” as they detonated two flash-bangs and breached the door. A grand jury declined to indict them, said District Attorney James R. Lestage.

But since the deputies were not equipped with cameras, there is no way to really know what happened, and the uncertainty has bred suspicion and litigation. Crime scene photographs show Mr. Senegal splayed out on the floor, blood pooling near his head, the gun about three feet from his sneakers. Derrick Kee, a lawyer who has filed a lawsuit on behalf of Mr. Senegal’s widow, suggests that it could have been planted.

The drug seizure was not insubstantial: 22 ounces of marijuana, about 20 grams of methamphetamine, a gram of cocaine, 34 prescription pills and $38.42 found in a Nesquik jar, according to a crime lab report.

Mr. Kee does not acknowledge that Mr. Senegal was dealing, but said it still would not justify his death. “These warrants introduce volatile circumstances into nonvolatile situations,” he said. “They should be issued only under the most unique circumstances.”

Officers Are Divided

Some SWAT veterans find it confounding that many police agencies remain so devoted to dynamic entry. The tactic is far from universally embraced, and a number of departments have retired or restricted its use over the years, often after a bad experience.

The National Tactical Officers Association, which might be expected to mount the most ardent defense, has long called for using dynamic entry sparingly. Robert Chabali, the group’s chairman from 2012 to 2015, goes so far as to recommend that it never be used to serve narcotics warrants.

“It just makes no sense,” said Mr. Chabali, a SWAT veteran who retired as assistant chief of the Dayton, Ohio, Police Department in 2015. “Why would you run into a gunfight? If we are going to risk our lives, we risk them for a hostage, for a citizen, for a fellow officer. You definitely don’t go in and risk your life for drugs.”

Another former chairman of the association, Phil Hansen, said SWAT teams tended to use dynamic entry as “a one-size-fits-all solution to tactical problems.” As commander of the Police Department in Santa Maria, Calif., and before that a longtime SWAT leader for the Los Angeles County Sheriff’s Department, he said it seemed foolhardy to move so aggressively in a state that voted in November to legalize recreational marijuana.

“Why am I risking people’s lives to save an ounce of something that they’re bringing in by the freighter every year?” he asked.

But officials with other law enforcement groups reject such absolutist approaches.

“If you want to take the position that narcotics laws in this country should not be enforced, then O.K., yeah,” said Sheriff Greg Champagne of St. Charles Parish in Louisiana, the president of the National Sheriffs’ Association. “That’s not the position of the law enforcement people around the country that I know. If you’re going to make narcotics cases you need to have evidence, and search warrants are how you get it.”

Sheriff Champagne said his deputies looked for opportunities to detain suspects on the street or in cars. Even so, he said, “there are times we just have to go in.”

“There’s an argument that no-knock warrants can actually be safer for residents and officers because a well-trained SWAT team can neutralize a situation in seconds and minimize the chance for hostage-takings and standoffs,” he added. “You can always point to the one bad case, but look at the thousands of cases where a no-knock warrant was executed without injury and heroin is seized. How many lives are saved because we got it off the street?”

No-knock warrants, said Bob Bushman, president of the National Narcotic Officers’ Associations’ Coalition, are “a tool that should stay in the toolbox.”

“There are some times,” he said, “that if you’re going to bring an investigation to a head the way to do it is with a no-knock.”

Clearly there are other factors that contribute to the tactic’s staying power. Some of it, according to long-term observers, derives from the adrenalized, hypermasculine, militaristic ethos of SWAT.

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“It’s culturally intoxicating, a rush,” said Dr. Kraska, the criminologist. “It involves dressing up in body armor and provocative face coverings and enhanced-hearing sets, a cyborg 21st-century kind of appeal. And instead of sitting around and waiting for something to happen twice or three times a year, you can go out and generate it.”

That culture is reinforced by a cottage industry of tactical training contractors, many of them veterans of the Iraq or Afghanistan wars, who are hired by police departments to keep SWAT teams up to date.

“For them, collateral damage is something you try to avoid but it’s not a deal breaker,” Commander Hansen said. “That doesn’t translate well for police work. If you’re in the military and told to clear a block of houses in a half-hour, you’re going to do it quickly by kicking in doors and throwing grenades. It’s a whole different theater of operations.”

Another potential factor is the incentive sometimes provided by asset forfeiture laws when contraband or drug proceeds are found in a residence. Revenue generated by those seizures typically reverts back to law enforcement agencies.

Connor Boyack, president of the Libertas Institute in Utah, said that was one of the rationales behind his state’s recent ban on forcible entry in drug possession cases. In 2015 when the new law passed, search warrant executions accounted for 29 percent of all forfeitures, according to a state report.

“We feel strongly that a lot of this is financial motive, not to keep the community safe,” said Mr. Boyack, whose libertarian-leaning group advocated for the restriction.

Further inducement has come from the Defense Department’s excess property program, which has distributed more than $6 billion in military vehicles, weapons and other equipment to law enforcement agencies since 1997. Until last May, the Pentagon required that any transferred equipment be “placed into use within one year of receipt.”

The Obama administration ended that requirement after a larger review of the so-called 1033 program, which was prompted by the police response to the 2014 civil unrest in Ferguson, Mo. President Trump has yet to act on a campaign pledge to rescind an executive order signed by President Barack Obama in 2015 that limited the kinds of equipment offered by the government. It is unclear whether he would reinstate the one-year rule.

‘Why Didn’t You Knock?’

As SWAT officers administered first aid to Bou Bou Phonesavanh, other agents detained his parents — Bounkham and Alecia Phonesavanh — and their three other children, ages 3 to 7.

“You know why we’re here,” an officer barked at Mr. Phonesavanh.

He didn’t. “Why didn’t you knock on the door?” he asked.

Elsewhere in the house, the agents came upon Mr. Phonesavanh’s sister, Amanda Thonetheva, who owned the place, as well as her boyfriend, her grandson and one of her sons. They did not find her other son, 30-year-old Wanis, who no longer lived there but dropped by at times. Nor did they find guns or drugs beyond some meth residue in a glass pipe. Later that night, deputies arrested Wanis at another address.

The Phonesavanhs had already suffered their share of misfortune. Earlier that year, the family’s house in Janesville, Wis., had burned down. They stayed in a motel as long as they could afford it, then lived for two weeks in their 11-year-old Chrysler Town & Country minivan.

They drove to Georgia when Mr. Phonesavanh’s sister offered the room in her garage. Seven weeks later, after struggling to find work, they were preparing to drive back to Wisconsin.

Remarkably, Bou Bou survived the explosion after being sped to a hospital in Atlanta. Now 4, he underwent his 15th surgery late last year, with more to come, his mother said. “The nightmares are still there,” she said, “several times a week. When he wakes up he’s usually sweating and holding his face.” She said all of her children became scared when they saw a police officer or security guard.

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The Phonesavanhs, who have returned to Janesville, received $3.6 million in settlements to the federal lawsuit they filed against the traumatized members of the drug and SWAT teams. The payments were made through government insurance policies purchased with taxpayer funds. All but $200,000, Ms. Phonesavanh said, has been spent on medical and legal bills.

“Things are still quite the struggle,” she said. “They didn’t mean to hurt my son, but they could’ve done a lot more to prevent this.”

A Habersham County grand jury issued a stinging report, but found no criminal negligence and declined to indict any of the participants. Federal prosecutors then won an indictment of Deputy Autry for violating Bou Bou’s civil rights, but she was acquitted after a weeklong trial. The jury accepted the defense’s assertion that the mistakes made by the former deputy, who had resigned, were unintentional.

In their closing arguments, opposing lawyers found common ground in their criticism of no-knock searches.

The prosecutor, Assistant United States Attorney William L. McKinnon Jr., called the tactic “probably the most intrusive contact that any citizen could have with the government.” He got no dispute from one of Deputy Autry’s lawyers, Michael J. Trost. “There’s a pattern of excess in the ways search warrants are executed,” he told the jury. “That’s what led to the injuries to this child.”

Read the second installment in this series.