The Los Angeles County Sheriff’s Department on Tuesday admitted that it killed an innocent man. “No question this is a terribly devastating event,” Capt. Steve Katz said during a news conference at which he explained that Donnell Thompson, a 27-year-old unarmed black man who was gunned down by an officer late last month, was not a second suspect in a carjacking as the police had initially claimed.

The story of Thompson’s death is a complicated one. It involves mistaken identity, an aggressive militarized police response, and miscommunication. Still, the simple and short version is this: L.A. police mistook one black man for another and took his life. The fact that history suggests this killing will eventually be deemed “justified” by officials makes it all the more maddening; it is a reminder that a criminal justice system built on the premise of innocence until proven guilt means little in the practice of standard police procedure.

So how did police end up coming to open fire on Thompson, who had no criminal record and reportedly took classes for the mentally disabled at a nearby community college? According to police, the story begins in the early hours of July 28 when a different man, Robert Alexander, stole a car at gunpoint in Los Angeles, opened fire at officers during an ensuing car chase in Compton, and eventually fled on foot into the neighborhood, where he was ultimately taken into custody at around 5 a.m. Less than a minute later—and while officers were still working to ID Alexander and connect him to the carjacking—a resident a few blocks away called 911 to report a man lying in his front yard. I’ll let the Los Angeles Times take it from there:

Meanwhile, the deputy who arrived at West Stockwell Street noticed that the man in the frontyard matched the description of the carjacking suspect: a black man 20 to 30 years old, wearing dark pants or shorts and a basketball jersey. He called over the radio that he believed he had found the carjacking suspect who had exchanged gunfire with deputies and that the man, lying motionless on the ground, may have been shot. “I could see why they were thinking, ‘Yeah, that was our guy,’” homicide Lt. John Corina said.

Thompson did not respond to commands to show his hands, to stand up or to put his hands behind his head, sheriff’s officials said. SWAT deputies arrived and repeated the commands, but Thompson remained motionless, lying on his side with his left hand underneath his head and his right hand under his midsection, Katz and Corina said. Even when the SWAT deputies set off “flash-bang” explosives, Thompson was unresponsive. Only when he was hit with foam bullets did he rouse himself, pushing himself onto his feet and “bolting” toward an armored vehicle, Corina said.

A SWAT deputy, who was in the turret of an armored vehicle on the street, then opened fire, striking Thompson twice in the upper torso. He was pronounced dead at the scene.

Deputies originally suggested that they saw Thompson’s right hand near his waistband while he was running, but the sheriff’s office later determined he had been unarmed. On Tuesday, Katz said that it was possible that Thompson had been unconscious prior to being hit with the rubber bullets. His family, meanwhile, said Thompson was likely just confused and afraid, and a lawyer representing the family told the Huffington Post that there is no evidence that Thompson “was in any way acting in an aggressive manner or in any way demonstrating that he posed a threat to anyone.”

Authorities are promising a “thorough” and “complete” investigation into the shooting, but Thompson’s family says that’s not enough—they want the officer who pulled the trigger, who has not been identified publicly, to face charges. That is unlikely, though. Our laws give officers broad leeway to use force, either when they fear their lives are in danger or when they are making an arrest. The Supreme Court cemented the scope of that authority in 1989’s Graham v. Connor, a case involving police officers that apparently mistook a diabetic who was behaving erratically due to his low blood sugar for a belligerent drunk. “The ‘reasonableness’ of a particular use of force must be judged from the perspective of a reasonable officer on the scene, rather than with the 20/20 vision of hindsight,” Chief Justice William Rehnquist wrote in the opinion.

In short, according to the U.S. criminal justice system, it’s not whether the officer is objectively correct when he uses force; it’s whether he subjectively believed that he was right in the moment he did. Based on the official version of Thompson’s death, it is certainly possible that the officer in question believed in the moment that he or she had no other choice but to pull the trigger. That system is clearly flawed and in desperate need of reform—cops, for example, are particularly ill equipped to deal with those with a mental disability or a diminished mental capacity, as Thompson’s family suggests he had. But in this case, as in others, an equally pressing question was why the officer who ultimately took an innocent life was perched atop an armored vehicle to begin with.

Elsewhere in Slate: A Better Way to Pursue Justice When Cops Kill in the Line of Duty