GREENSBORO, Md. — It was a warm Saturday evening when Jennell Black heard a commotion outside her mobile home on Maryland’s Eastern Shore. Outside her door, four men were holding her 19-year-old son face down, pinning his shoulder, legs and arms. One of them, who turned out to be an off-duty police chief from a nearby town, was lying on top of him.
“That’s Anton,” Ms. Black said in disbelief. Then she raised her voice: “Anton!”
She watched helplessly as Anton Black, a former star high school athlete with a nascent modeling career, struggled and then became unresponsive. The officers told Ms. Black that her son was having a mental health emergency and would be taken to a hospital, not to jail.
They assured her that he was breathing and had a pulse. But when they finally sat him up, she could tell something was gravely wrong. “He’s turning dark,” she said. He never regained consciousness.
It was 2018, two years before George Floyd was killed in Minneapolis after similar treatment, with two officers holding down his lower body and one with a knee on his neck. Mr. Floyd was pinned face down for nine and a half minutes, Mr. Black for more than six.
An autopsy report released four months later blamed Mr. Black’s death on congenital heart abnormalities. It classified the death as an accident and said there was no evidence that the police officers’ actions had played a role. “It was horrible,” said LaToya Holley, one of Mr. Black’s sisters, adding that his sports physicals had never revealed any heart problems. “We didn’t imagine at all that it would read the way it did.”
The medical examiner’s findings thrust the Black family into one of the most contentious issues in American policing today: unarmed people who die after being restrained. The debate, playing out in autopsy rooms, courthouses and police training sessions across the country, hinges on a body of research, conducted primarily by a small group of doctors in San Diego over the last three decades, that concludes that prone restraint is safe and that deaths like those of Mr. Black and Mr. Floyd are caused by factors that have nothing to do with police tactics.
The research, which began as a defense for four California sheriff’s deputies involved in a fatal encounter, has easily dominated the peer-reviewed literature on this niche topic. It is widely cited by medical examiners, including David R. Fowler, who signed off on the autopsy of Mr. Black. It has been used to defend the police in scores of cases across the country, including in the deaths of Mr. Floyd in Minneapolis, Daniel Prude in Rochester and Humberto Martinez in Pittsburg, Calif., whose family was awarded $7.3 million in a settlement last year.
The precise number of in-custody death investigations the San Diego research has influenced is impossible to know, in part because it has been used to ensure that such cases never reach a courtroom. A joint investigation by news stations in Minneapolis and Denver counted more than 113 police prone restraint deaths since 2010, costing taxpayers $70 million in wrongful death payouts. Criminal charges against officers in such cases are exceedingly rare.
But as fatal police encounters draw increasing scrutiny, so has the research — a growing chorus of experts argues that it is flawed and has been too broadly applied. The studies do not, and ethically cannot, replicate the stress and violence of real police pursuits, and critics say they fail to take into account the physical vulnerabilities, including drug use, obesity and pre-existing health conditions, of many people who wind up being subdued.
On Thursday, a leading medical journal published a new study that found that fatal police violence is frequently misclassified, in part because medical examiners identify other reasons for deaths that occur in police custody.
Some critics of the restraint research had hoped its influence would crater after this year’s murder trial of Derek Chauvin in the death of Mr. Floyd. Mr. Chauvin’s lawyers tried to use the research to bolster their defense, but the prosecution’s star expert witness dismissed it as “highly misleading,” explaining in granular detail how the position and weight of the officers forced Mr. Floyd to fight for air. Mr. Chauvin was convicted.
“I thought, ‘This is it — they’ll never ever be able to defend prone restraint again,’” said Alon Steinberg, chief of cardiology at Community Memorial Hospital in Ventura, Calif., and the author of a recent paper on prone restraint and cardiac arrest. But the San Diego studies have continued to mislead officers into believing the technique is safe, he said, and have given the legal system a means to excuse their actions.
“People are dying all the time, and we’re not doing anything about it,” said Dr. Steinberg, who has worked as a consultant in cases against police officers. “I want to shout it out to everyone: Let’s stop this right now.”
After the autopsy report on Mr. Black, the local prosecutor said he would not pursue criminal charges against the police officers involved. When the family filed a lawsuit, the officers responded that they could not have killed Mr. Black, because the research had “thoroughly debunked” prone restraint as a cause of arrest-related death.
It is not known whether the deputy medical examiner who performed the autopsy in Mr. Black’s case, Russell Alexander, relied on the San Diego studies when he reached his conclusions. The Office of the Chief Medical Examiner declined to comment on the case because it is the subject of litigation.
But Dr. Alexander’s boss at the time, Dr. Fowler, has cited the studies in his published research and discussed them at length when he testified for the defense at the Chauvin trial. After complaints that Dr. Fowler’s testimony was baseless and disingenuous, the Maryland attorney general announced a review of in-custody deaths handled during his tenure — including, presumably, Mr. Black’s.
Thestudies began after the death of a 35-year-old man named Daniel Price, who was high on methamphetamine when he was subdued by four San Diego sheriff’s deputies in June 1994. They pepper-sprayed him, wrestled him to the 134-degree pavement and hogtied him. The autopsy cited “maximum restraint in a prone position by law enforcement.”
When Mr. Price’s family filed a lawsuit, the county turned to doctors at the San Diego Medical Center for help.
At the time, it was widely accepted that prone restraint, used to subdue combative or disorderly people, could be lethal. It compresses the torso and restricts breathing, potentially leading to what is referred to as positional asphyxia or restraint asphyxia.
Many police departments had already banned the most extreme form of prone restraint, hogtying, and in 1995 the Justice Department warned of its dangers.
The San Diego doctors proposed a study, paid for in part by the county, that would measure the effects of hogtying in controlled laboratory conditions.
The study’s authors — Tom Neuman, Gary M. Vilke, Theodore C. Chan and Jack L. Clausen — found that putting someone into a prone position restricted lung capacity and that using the most extreme form of prone restraint, hogtying, could reduce breathing capacity by as much as 23 percent. But the effects, they said, were not big enough to be “clinically relevant.”
A federal judge ruled in favor of the county, saying the study had shown the dangers of hogtying to be “fictitious.” But Mr. Price himself would not have qualified for the study, which was done on healthy, drug-free subjects.
In the years since, the San Diego doctors and other researchers have gone on to conduct a host of additional studies on prone restraint, continuing to find that it poses no inherent dangers even when 225-pound weights were placed on subjects’ backs to simulate the weight of officers’ bodies. Several of the doctors have received hundreds of thousands of dollars for testifying in defense of police officers.
Many departments still teach officers that prone restraint is dangerous and should be used only sparingly. But the San Diego research over the years has trickled into police training, and one law enforcement news website calls restraint asphyxia a “myth.”
In December, Dr. Vilke testified before a grand jury considering charges in the death of Daniel Prude, who had died after an encounter with the police in Rochester, N.Y. The officers placed Mr. Prude, who had taken P.C.P. and had been running, in a mesh hood that covered his mouth and nose, and then held his face to the street.
Dr. Vilke said that people in Mr. Prude’s excited state often die with or without restraint, and that being restrained from further exertion had actually helped him. Mentioning his prone restraint studies, he said, “Nobody dies, they breathe well, they move oxygen levels well. It doesn’t kill people.” None of the officers were charged.
Martin Tobin, a renowned specialist in the mechanics of breathing, testified during the Chauvin trial that the studies used the “wrong yardstick” to conclude that the reductions in lung capacity were not significant, and that placing large weights on a subject’s back would fall short of replicating the pressure of an officer’s knee by a factor of 10.
Experts agree that prone restraint may well be safe for most people. But they said that drug use, mental crisis, pre-existing conditions, obesity and exertion can increase the need for oxygen to the point where even a small restriction can be fatal.
“They did a service to medical knowledge by publishing these studies,” said Judy Melinek, a prominent American forensic pathologist practicing in New Zealand. “But the conclusions that they reached in their papers, and that they continue to advocate for in their testimony, are a lot more far-reaching than the data that they collected can support.”
In a statement to The New York Times, Drs. Chan, Vilke and Neuman (Dr. Clausen is retired) wrote that real-world factors like drug use and exertion “have not been shown to negatively impact ventilation, thus would not be expected to increase risk for asphyxiation, regardless of body position.”
Sudden deaths in custody, they wrote, are best understood “through a variety of robust and diverse scientific methods and investigations,” rather than “conjecture and media sensationalism.”
Michael Freeman, a forensic epidemiologist who has testified for families in police custody death cases, said the San Diego studies have had the effect of pushing medical examiners to look past the obvious to more far-fetched explanations. “It can’t be positional asphyxia, because look, there’s a study,” said Dr. Freeman, one of the experts who will help steer the review of in-custody deaths in Maryland.
On the day Anton Black died, a police officer confronted him after a 911 call reporting that an older boy had a younger boy in a headlock. Ten days earlier, Mr. Black had left the psychiatric ward of a hospital with a diagnosis of bipolar disorder. The younger boy, a family friend, told the officer, Thomas Webster III, that Mr. Black was “schizophrenic.” Mr. Black turned and jogged away, and the subsequent chase ended with him restrained at his mother’s doorstep.
The deputy medical examiner who performed the autopsy, Russell Alexander, dismissed the idea that the officers caused Mr. Black’s death, instead attributing it to cardiac arrest caused by two congenital heart abnormalities.
“It is likely that the stress of his struggle contributed to his death,” Dr. Alexander wrote. “However, no evidence was found that restraint by law enforcement directly caused or significantly contributed to the decedent’s death; in particular, no evidence was found that restraint led to the decedent being asphyxiated.”
That statement was misleading, according to some independent forensics experts who reviewed the autopsy report: It failed to account for the duration and intensity of restraint seen on body camera video, as well as signs of possible asphyxiation, including pinpoint blood spots in Mr. Black’s eyes and electrocardiogram readings suggesting that he was having a type of cardiac arrest associated with oxygen deprivation.
In the end, Dr. Freeman said, the deputy medical examiner’s report focused on abnormalities that were statistically very unlikely to have caused a 19-year-old man’s heart to stop.
“He has not examined the probable, and he has latched onto the highly improbable,” Dr. Freeman said. “This is a report that’s designed to exculpate the cops.”