Protesters in the Californian capital have been on the streets almost every day since Clark's death on March 18, demanding that the city leadership fire the two officers involved.
Mr. Clark's family has accused the police of attempting to cover up the misconduct of their officials and decided to autopsy.
Video showed that officials shouted at Mr. Clark minutes after shooting. "We need to know if you're O.K.," an officer shouted about three minutes after the shootings ended. "We have to bring you doctors, but we can not go over to get you help unless we know you have no weapon."
Dr. Omalu said the autopsy indicated that Mr. Clark lived three to ten minutes after the shooting, which led to the question of the time it took to get him treated. Medical help arrived about six minutes after the shootout.
In their first report, the police department said Mr. Clark had "approached the officers" while holding what they considered a firearm. However, in the body photographs provided by the police, it is unclear which direction Mr. Clark is facing, and the family lawyer Benjamin Crump said the independent autopsy contradicts the police's claim to be a threat.
Mr. Crump said the results proved that Mr. Clark did not approach the officers in a threatening manner when they opened the fire.
"These independent autopsy findings contradict the police census we were told," he said. "This independent autopsy confirms that Stephon was not a threat to the police and was killed in yet another pointless police order under increasingly questionable circumstances."
External experts who investigated the case say it will be difficult to determine if the officers could be held criminally responsible. The Supreme Court has sided with the police in fatal shots when it is revealed that officers reasonably believe that their lives are in danger.
Justin Nix, a police officer at the University of Nebraska Omaha, said, "Any police shooting in front of the camera will look bad, but if the guy lies on his stomach and continues to take pictures, many people will be upset."  Mr. Nix agreed with the autopsy, undercut the police events, but said, "He's a little on her side, and it's possible they believed he was still reaching for something they thought was a weapon."
David P Harris, a professor at the Law Faculty of the University of Pittsburgh, who deals with police accountability, said the officers were at a disadvantage. They relied on information about the suspect from a police helicopter circling above them.
However, as soon as they face the suspect, the officers order Mr. Clark to "show" his hands instead of raising his hands that Mr. Clark might have
But he said, when the officers noticed Mr. Clark was armed and approaching, they were trained to shoot. "It's not clear that they could have done something different," he said.
Although the shots were problematic for Mr. Clark's back, they were "not enough to seal a negative verdict," he said. Partly because "the body of the victim could have turned after the shooting started, and it's still unclear if they could see he's turned."
Sacramento police chief Daniel Hahn asked the California Department of Justice for help earlier this week, led by Attorney General Xavier Becerra, as an independent party to investigate the department. Mr. Hahn said he hopes this move will reassure residents that the investigation is impartial.
The episode began when two officers were sent to the Meadowview neighborhood in South Sacramento to investigate a report that someone was breaking car windows. A county sheriff's deputy helicopter joined the search and hovered over him, once telling the officers that a suspect had taken a crowbar.
The officers finally saw Mr. Clark, who seems to have run into the garden of his grandmother. In a body-camera video, an officer will repeatedly yell the word "weapon" and open the fire almost immediately. No weapon was found on Mr. Clark's body. the only object found was his cell phone.
After other officers arrived, the two policemen involved in the shooting dampened the sound on their body cameras as they discussed what had happened, which also caused criticism.
Hundreds of mourners gathered at his funeral on Thursday, including Rev. Al Sharpton and others from the Blacks Lives Matter movement. Mr. Clark's brother, Stevante, asked followers not to forget his brother. Protests against the shooting are scheduled for Saturday.
Dr. Omalu is widely attributed to the discovery of chronic traumatic encephalopathy or C.T.E., irreversible brain damage caused by repeated head-on bouts associated with football matches. His fight with the National Football League was featured in the 2015 film Concussion. Until the end of last year, he was chief medical examiner of San Joaquin County, California, but after accusing the sheriff, quit police custody cases
He said he could not determine if Mr. Clark had survived if he had received medical help faster, but "every minute you wait reduces the likelihood of survival."
Mr. Crump said he expected the authorities to push back the results of the autopsy.
Continue reading the main story