Dr. Bennet Omalu conducted an autopsy days after Clark was killed by police. He told reporters that his examination showed that Clark was hit by eight bullets, and all but one entered while his back was facing police.
The eighth bullet that hit Clark entered his left thigh from a forward-facing and likely came while he was on the ground and had already been shot multiple times, Omalu said.
"That he was assailing the officers, meaning he was facing the officers, is inconsistent with the prevailing forensic evidence" as documented in the autopsy, he said.
The county coroner's official autopsy results are not expected to be completed for several weeks.
Clark, 22, was fatally shot in his grandmother's backyard on March 18 by Sacramento police, who were looking for a vandal in the neighborhood. More than 20 shots were fired in the incident.
Clark was holding a cellphone. No weapon was found.
The independent autopsy finding raises new questions about Clark's shooting earlier this month. Police have released videos showing the incident but have urged the public not to pass judgment on officers until the investigation is complete.
"It's very simple. The narrative that's been put forth is they had to open fire because he was charging toward them," said civil rights litigator Ben Crump, who is representing Clark's family. Yet the autopsy shows, Crump said, "all of the bullets were from behind."
Omalu said it appeared the coroner did not seek to determine the pathway of the bullets, key to determining the sequence of the bullets.
His family has disputed police accounts of what led to the shooting, which inspired days of protests in the state capital and made national headlines.
Clark was buried after a funeral in Sacramento on Thursday attended by hundreds of mourners, including the Rev. Al Sharpton.
"We are here to say that we're going to stand with Stephon Clark and the leaders of this family. ... This is about justice," Sharpton said. "This is about standing with people with courage."
Clark's shooting is under investigation by city police, with oversight by the state Department of Justice.
Sacramento's mayor has called Clark's death "wrong," but said he cannot pass judgment on the officers' actions until that review is complete.
On Wednesday, a police spokesman said Clark remained the sole suspect in break-ins of vehicles and what a sheriff's deputy said was the attempted break-in of a home. It was a call about those incidents that sent police to the neighborhood the night Clark was shot.
A Sacramento County Sheriff's Department helicopter spotted a man in a backyard and directed police toward him, authorities said. Deputies told officers that the man had picked up a "toolbar" and broken the window of a home.
The man was seen running south, toward the front of the house, where he stopped and looked into another car, police said. Police body camera footage shows officers intercepted Clark in the backyard of his grandmother's house, and one of them yelled "gun!" as he turned a corner and saw Clark. The officer ducked back momentarily, then looked around the corner again and, shouting "gun! gun! gun!" began rapid fire. His partner then joined in the shooting.
The officers told other police who arrived on the scene they thought Clark was pointing something at them. "It looked like a gun from our perspective."
The object found when they rolled over Clark's bleeding body was a white cellphone.
Clark was pronounced dead at the scene. Police video shows it was several minutes until officers approached Clark's body. They then handcuffed him before appearing to attempt resuscitation. Omalu said Friday it would have taken from three to 10 minutes for such wounds to kill him. "Every minute you wait decreases the probability of survival," he said.
In police videos, a sergeant pulls the two officers to the side and a voice says "Hey, mute," before the sound cuts off, indicating that the audio recording had been stopped.
Sacramento's police chief said the request to mute "builds suspicion" and is part of the investigation.
Clark's family has denied he had anything do with any break-ins. His grandmother said she heard gunfire in her backyard and asked her husband to call police. But it turned out the police were already there, having just fatally shot Clark.
"Why didn't you shoot him in the arm? Shoot him in the legs? Send in dogs? Send in a Taser? Why? Why?" Sequita Thompson said.
Sacramento police have refused to name the two officers who shot Clark, but they were identified by an area civil rights attorney who saw their names on unedited video captured by the body cameras they wore.
A Sacramento Bee article noted that one of the officers, who is African-American, joined the department in 2016 after prior work with a police department in Mississippi.