Stephon Clark, the unarmed man whose killing by police has sparked days of protest, was repeatedly shot in the back, according to an independent autopsy requested by his family.

Lawyers for Mr Clark’s family said that the results of the report, released at a news conference, undercut statements from the Californian city’s police over the shooting earlier this month. The Sacramento police department had said that officers responding to reports of vehicle break-ins encountered Mr Clark “facing them” and he began to “advance forward with his arms extended” while “holding an object in his hands”.

Officers opened fire 20 times, killing Mr Clark in his grandparents’ backyard. Police officials have said the officers believed Mr Clark was armed – one of them can be heard shouting “Gun”! in footage of the incident – but they only found a mobile phone in searching the scene.

Dr Bennet Omalu determined Mr Clark’s death “was not instantaneous” and that the 22-year-old took between three and 10 minutes to die. Police officers waited for a number of minutes to approach him after the shooting.

“The proposition that has been presented that he was assailing the officers – meaning he was facing the officers – is inconsistent with the prevailing forensic evidence”, Dr Omalu, best known for studying the impact of concussions on American football players, said.

Of the eight gun shot wounds on Mr Clark’s body, Dr Omalu said, six were in his back and one on the side of his body but towards the back.

“You could reasonably conclude that he received seven gunshot wounds from his back”, Dr Omalu said. All seven of those wounds were severe enough to be fatal, Dr Omalu added.

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Attorneys representing were even more forthright about what the autopsy meant for the explanation that the officers believed they were under threat.

“It contradicts the narrative that had been put forth by the police”, attorney Benjamin Crump said. “The narrative that had been put forth was they had to open fire because he was charging at them”.

The Sacramento Police Department said it had not been provided with the official report from the Sacramento County Coroner’s office over the death of Stephan Clark and that further comment prior to the release of that report, and the completion of investigations into the case ”would be inappropriate at this time”.​

Al Sharpton gives eulogy at Stephon Clark funeral

As anger and grief over Mr Clark’s killing have convulsed Sacramento, activists have called for the officers who shot Mr Clark to be prosecuted. Attorney Dale Galipo said the autopsy’s finding “will be very important to that determination” of whether the officers face charges.

Seeking to reassure the community that the investigation of the shooting will be fair and thorough, California attorney general Xavier Becerra announced this week that he would oversee the probe.