Stephon Clark, the unarmed black man who was fatally shot last week by Sacramento police officers, was struck eight times, mostly in his back, according to an independent autopsy released Friday, raising significant questions about the police account that he was a threat to officers when he was hit.

The autopsy — commissioned by the family of Mr. Clark, 22, and conducted by Dr. Bennet Omalu, a private medical examiner — showed that he was shot three times in his lower back, twice near his right shoulder, once in his neck and once under an armpit. He was also shot in the leg. The neck wound was from the side, the doctor found, and he said that while the shot to the leg hit Mr. Clark in the front, it appeared to have been fired after he was already falling.

“He was shot from the back,” Dr. Omalu said Friday at a news conference. Standing next to diagrams of the findings, he said that seven of the shots could have had a “fatal capacity.” He described severe damage to Mr. Clark’s body, including a shattered vertebrae, a collapsed lung and an arm broken into “tiny bits.”

“He bled massively,” said Dr. Omalu, who became nationally known for his fight with the National Football League over head injuries to its players.