The two officers were responding to a 911 call about a man breaking vehicle windows when they encountered, then killed, Clark, an unarmed black man.
Video released by the Sacramento Police Department depicts a frantic foot pursuit through darkened streets pierced by white slivers of police flashlight.
The officers spot Clark approaching a house and shout: "Show me your hands! Stop! Stop!"
Clark is seen running, and the two officers round the corner of the house and find him under a covered patio.
An infrared camera on an overhead helicopter briefly loses sight of Clark.
"Show me your hands! Gun!" an officer shouts and ducks behind the wall in a fraction of a second.
The helicopter footage shows one of the officers appearing to grab his partner to pull him to cover.
Clark steps toward the officers. Behind the wall, one of the officers issues another command. "Show me your hands!" And then: "Gun, gun, gun!"
Both officers open fire. Sparks from the bullets light up the helicopter's infrared camera in sharp white pops.
The sequence, from the first glimpse of Clark on the patio to the first gunshot, unfolds in about six seconds.
The officers are never heard identifying themselves as police before fatally shooting Clark.
The gun officers thought Clark had in his hand was actually a white iPhone.
"He was at the wrong place at the wrong time in his own back yard?" his grandmother, Sequita Thompson, told the Sacramento Bee. "C'mon, now, they didn't have to do that."
The Rev. Al Sharpton, who has spoken with Clark's mother and plans to travel to California to meet with the family, said in a statement: "It is an atrocity that an unarmed young man was shot at 20 times in his own backyard and shows the urgent need in these times for intervention against police misconduct."
Sharpton added: "We will call for a complete and thorough investigation into this young man's death."
The Sacramento Police Department said the man they believed was breaking windows was the same man the officers killed in a hail of gunfire, identified by the 911 caller as a thin 6-foot male wearing a black hoodie and dark pants.
Police have yet to identify Clark as the suspect or victim.
But Thompson and other relatives identified him to media using variations of his name, Stephon and Stephan. (Public records list him as Stephon Clark, 22.)
Thompson disputes the police department's version of events.
Her grandson was short, not 6 feet, she said in a video produced by the Bee. She believes another suspect was smashing windows, and that Clark was in the back yard at the wrong time.
Their doorbell is broken, and relatives often tap on the back window for someone to open the garage door, the family told the newspaper. Clark was staying at his grandmother's home at the time he was killed.
The gunfire startled her that night, she told the Bee.
"The only thing that I heard was pow, pow, pow, pow, and I got to the ground," she said in the Bee's video.
She said she began to suspect the police description of a dead person in her yard was a member of the family.
"I told the officers, 'You guys are murderers. Murderers,' " Thompson cried out. "You took him away from his kids."
The family said Clark had two young sons, Cairo and Aiden, and a fiancee, Salena Manni, the Associated Press reported.
Thompson did not immediately respond to a request for comment.
The narrative of the Sunday night shooting released by authorities tells a short, grim story.
The helicopter observed a suspect picking up a "toolbar" and breaking a window to a house after 9 p.m. Sunday night.
The Bee reported it was the sliding glass door belonging to a neighbor.
Authorities said the suspect then ran and looked into a car.
Police in the helicopter guided officers on the ground to the front yard of Thompson's house as Clark was coming from the back.
They met in the middle, and soon Clark was dead.
An analysis by The Washington Post found that 987 people were killed by police last year - 68 of them unarmed. Of those unarmed victims, 30 were white, 20 were black and 13 were Hispanic, showing an overrepresentation of African Americans among the total population. Five of the remaining fatalities were of unknown or other race.
At least 230 people have been killed by police this year, according to The Post's database on fatal force.
"I know there could have been another way; he didn't have to die," Clark's brother Stevante told CBS News.
"You're going to know his name forever," he added before reciting the names of several black men who were killed by police: "You're going to remember it, like, how you know . . . Mike Brown, Eric Garner, Tamir Rice. You're going to know him. You're going to remember this."
Sacramento Mayor Darrell Steinberg offered his condolences to Clark's family and said in a statement that he was "heartbroken" for the city.
"The questions raised by the community and council members are appropriate and must be answered during the investigation," Steinberg said, though he noted that he had reviewed the police videos carefully and said: "Based on the videos alone, I cannot second-guess the split-second decisions of our officers and I'm not going to do that."
Clark is at least the sixth person shot and killed by the Sacramento Police Department since the beginning of 2015, according to a Post analysis: Five of them were black men; the other, a white man.
The October 2015 shooting of Adriene Ludd and the September 2017 shooting of Eric Arnold were the only two of the six fatal Sacramento police shootings in which the person killed was armed with a gun.
Police say Ludd fled after a traffic stop and fired at officers before he was killed.
Arnold, a suspect in a double homicide, shot two police officers before he was shot and killed.
Matt Coates was holding a plastic BB gun when he was shot and killed in May 2015; his girlfriend would later tell reporters that she had told the officers that the gun wasn't real. In two of the cases - the fatal shootings of Dazion Flenaugh and Joseph Mann - Sacramento police killed people alleged to have been armed with a knife.
Clark, it appears, was unarmed.
How many times Clark was shot is unknown, authorities said, pending the investigation. The officers are on paid administrative leave as the probe unfolds, officials said.
Detective Eddie Macauley of the Sacramento Police said Wednesday he was unsure what model of weapon the officers used, or if the 10 rounds each of them fired was the entire capacity of their magazines.
The police said five minutes passed before responding officers arrived to cuff Clark and render first aid, which ultimately proved futile: He died at the scene.
That timeline is not precise, according to the footage. Five minutes and 16 seconds pass between the radio call of shots fired and when officers snap on the cuffs. Several more seconds pass before someone on scene begins chest compressions.
A single sentence of department guidance on providing medical attention to suspects reads: "Officers shall provide first aid to injured parties if it can be done safely."
Some factors may affect how and if police render aid to someone they shoot, such as if they are resisting or if police think a weapon is present, said Macauley, the police detective.
In the video, the officers did not appear to be fearful of an attack once Clark was down.
He wasn't moving, an officer notes. One officer, showing no clear urgency to replenish his ammunition, waits a minute and a half before he reloads.
"Sir, can you move?" an arriving officer calls into the night at Clark, minutes after the shooting, telling him they cannot help unless they know he does not have a weapon.
Police allowed Clark's family to review the body camera video before it was publicly released - part of a departmental policy change, according to the Bee:
"Allowing family to see such videos before they are released to the public is part of a city policy adopted in late 2016 by the city of Sacramento after the fatal shooting by police of Joseph Mann, a mentally ill black man. Mann's shooting led to major changes in the department, including a requirement that all patrol officers wear body cameras.
"The changes also require police to release videos in "critical incidents" such as officer-involved shootings and deaths in custody within 30 days of the event. Sacramento police Chief Daniel Hahn, the city's first African American chief, has been releasing videos more quickly than the requirement and for a broader range of events than covered by the new law since taking over the department last summer."
"As soon as they did the command, they started shooting," Clark's aunt Saquois Durham told the Bee. "They said 'Put your hands up, gun,' and then they just let loose on my nephew."
Said Les Simmons, a pastor and community activist: "Even if he did what they say was done, at the end of the day it does not justify his life being taken."
Simmons called into question what was left off the released video, particularly at the end.
Before the video concludes, the two officers walk to the street, nearly seven minutes after the shooting.
Shimmering red and blue lights silhouette an approaching group of officers. Their faces are blurred.
"Hey mute?" an officer says. The audio goes silent, and shortly after, the videos end.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.