Stephon Clark, an unarmed black man who was killed by Sacramento police officers earlier this month, was shot eight times, with nearly all of the shots striking him from behind, according to private autopsy results announced Friday.

Dr. Bennet Omalu, announcing the results of his autopsy at a news conference in Sacramento, concluded that Clark was shot seven times from behind on the right side of the body and once from the front. He added that Clark's death was not instantaneous, but "took anywhere from three to 10 minutes.”

The autopsy was done at the request of Clark's family. The Sacramento County Coroner's Office has conducted an official autopsy but has not released it, saying it is waiting on final toxicology results. The office has confirmed that Clark died of multiple gunshot wounds.

Omalu said any one of the bullets striking Clark from behind could have killed him. "Each one of these bullets independently possessed a fatal capacity," said Omalu, who is known for identifying and describing the first instance of chronic traumatic encephalopathy, or CTE, in professional athletes.

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The first shot by Sacramento police officers hit Clark in the left side and back of the chest, with the bullet’s propulsion probably forcing him to spin around so that his back faced the officers completely, Omalu said. The shot fired from the front struck him in the lower part of his left thigh, indicating that Clark was shot while he was either on the ground or falling close to the ground, according to Omalu.

Video from a police helicopter shows Clark, 22, running from the officers, then crawling away after being shot.

Omalu, who conducted the autopsy on Tuesday, further concluded that Clark’s death was caused by a combination of blood loss, acute respiratory arrest and hypoxia, or a lack of oxygen reaching the brain.

According to police, two Sacramento officers fired 20 shots at Clark on the night of March 18 after responding to a 911 caller reporting a man smashing car windows. Believing Clark to be a suspect, officers chased him to his grandmother’s backyard with the help of an overhead helicopter from the Sacramento County Sheriff's Department. Body camera footage shows the two officers, who have not been identified, shouting at Clark to stop and show his hands before one of them yells “Gun! Gun! Gun!” and both officers open fire. Later, Clark was found to have only been carrying a cellphone.

“These findings from the independent autopsy contradict the police narrative that we’ve been told,” Benjamin Crump, an attorney representing the Clark family, said in a statement. “From the time this investigation began, statements provided by the Sacramento Police Department have proven to be self-serving, untrustworthy and unreliable. This independent autopsy affirms that Stephon was not a threat to police and was slain in another senseless police killing under increasingly questionable circumstances.”

In an interview on March 22, Police Chief Daniel Hahn told NBC News that officers who engaged Clark had feared for their lives before the shooting.

"The officers responded to a call and ultimately were able to see the subject they believed was responsible for the breaking into the cars that they were there for," said Hahn. "They felt their lives were in danger and they fired."

In a statement on Friday, Sacramento Mayor Darrell Steinberg said of Clark's killing: "From the moment we saw the video we knew the details of this horrific shooting were graphic and disturbing."

"We have an obligation to everyone involved, including the family of Stephon Clark, to wait for the full findings and results from the official autopsy and investigation," Steinberg said. He said his office would work aggressively to seek answers to questions the community is asking. “As important, we will aggressively seek appropriate change to the protocols and training that led to this unacceptable outcome,” the mayor said.

Sacramento police said in a statement Friday that they have not received the official report from the coroner’s office, and that the coroner's office "conducts an independent death investigation that is separate from the joint investigation being conducted by our agency and the California Department of Justice."

"We acknowledge the importance of this case to all in our community and we are committed to a thorough and comprehensive investigation," police said in the statement.