Lashunda Britt, a cousin of Stephan Clark, stands near where he was fatally shot by police in Sacramento.

US police say they saw an object in Stephan Clark's hand before firing 20 bullets that killed him in his own back yard.

The two officers were responding to a 911 call about a man breaking vehicle windows, and encountered Clark, 22. They believed he was holding a gun. It was a cellphone.

They each fired 10 rounds at Clark, striking him multiple times.

"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."

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Police said the man they believed was breaking windows in Sacramento was the same man they killed in a hail of gunfire, identified as a man over 182cm, the Sacramento Police Department said in a statement on Monday.

Thompson disputes that. Her grandson was short, she said in a video produced by the Bee. She believes another suspect was smashing windows, but Clark was in her back yard at the wrong time.

The door bell is broken, and family members tap on the back window for someone to open the garage door, the family told the paper. Clark was staying at his grandmother's at the time.

Gunfire startled her that night.

"The only thing that I heard was pow, pow, pow, pow, and I got to the ground," she said in the video. She said she began to suspect the police describing a dead person in her yard was a member of the family.

"I told the officers, 'You guys are murderers. Murderers'," Thompson cried out in the video. "You took him away from his kids." He had two young sons, Cairo and Aiden, the family said, and a fiancee, Salena Manni, the Associated Press reported.

Police have yet to identify Clark as the suspect or victim, but Thompson and Manni both identified him to media using variations of his name, Stephon and Stephan.

Thompson did not return a request for comment.

The narrative of the Sunday night shooting released by authorities tells a short, grim story.

Police were dispatched at 9.13pm Sunday, responding to a 911 call of a man breaking car windows and then hiding in a back yard in southern Sacramento. They arrived minutes later, along with a police helicopter flying overhead.

The helicopter observed a suspect picking up a "toolbar" and breaking a window to a house. The Bee reported it was the sliding glass door belonging to a neighbour. The suspect then ran and looked into a car, the statement said.

Police in the helicopter guided police on the ground to the front yard of Thompson's house as Clark was coming from the back. They met in the middle. Police commanded him to stop and show his hands. He ran toward the back yard, they said.

"The suspect turned and advanced toward the officers while holding an object which was extended in front of him. The officers believed the suspect was pointing a firearm at them," the statement read.

"Fearing for their safety, the officers fired their duty weapons striking the suspect multiple times."

How many times Clark was struck is unknown, pending the investigation.

Sacramento police Detective Eddie Macauley did not immediately know what model weapon the police used, or if the 10 rounds they each fired was the entire capacity of their magazines.

Authorities released some details about remaining moments of Clark's life.

The two officers on scene struck Clark with gunfire, but waited five minutes to render first aid after other officers arrived and Clark was handcuffed. He died at the scene.

A single, broad 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, like if they are resisting or if police believe a weapon is present, Macauley told The Washington Post on Wednesday.

An analysis by The Post found that 987 people were killed by police last year - 68 of them unarmed. Of those, 30 were white, 20 were black and 13 were Hispanic, showing an over representation of African Americans among the total population. Five of the remaining fatalities were unknown or other.

Macauley declined to say when the officers learned Clark did not have a gun or what Clark's demeanour was, citing the ongoing investigation.

Both officers were wearing body cams, Macauley said, and authorities are working to get the footage released. They are both on paid administrative leave, he said.

Sacramento mayor Darrell Steinberg in a statement Wednesday night offered his condolences to Clark's family and said he was "heartbroken" for the city.

He also said that he reviewed the videos carefully and, "based on the videos alone, I cannot second guess the split-second decisions of our officers and I'm not going to do that." A thorough investigation was necessary, he said, "before we can render any final conclusions."

"The questions raised by the community and council members are appropriate are appropriate and must be answered during the investigation," Steinberg wrote. "For instance, what are the protocols regarding use of force and for rendering emergency aid during officer involved shootings?"

The city enacted a regulation that requires police to release footage no later than 30 days after an officer kills a citizen. The policy has been in place since 2014, after police killed knife-wielding Joseph Mann after first trying to ram him with a cruiser. Police fired 18 rounds at him, striking 14 times.

"I think right now is a moment for our community," local pastor Les Simmons, who resigned from a police oversight commission after Mann's death, told the Bee following a Monday vigil and protest that included Black Lives Matter. "A moment to show real transparency."

The Bee reported that before Clark's death, 16 officer-involved killings occurred in Sacramento County since 2016.

Some of them are detailed on the Sacramento Police Department's transparency section of its website, right under a banner touting the department's tagline.

"Making Sacramento the safest big city in California," it reads.