Video shows 27-year-old Dillan Tabares confronting the officer and punching him, then reaching for his duty belt as the two struggled before seven shots were fired, the office said.
Tabares was pronounced dead shortly after the September morning shooting outside a Huntington Beach convenience store.
County prosecutors released a nearly 13 minute-video that includes footage from bystanders' cellphones and officers' body-worn cameras.
Huntington Beach police officer Eric Esparza, who fired the shots, can be heard telling another officer who arrived to assist him that Tabares had tried to grab his gun.
Investigators said Tabares' DNA was later found on the grip of the Esparza's pistol and his flashlight, which he appears to have snatched during the tussle.
It is not clear what prompted Esparza to stop Tabares in the first place. Esparza declined to provide a statement to investigators.
"When they just give us a statement of course that makes it easier, but they do have their rights and we can't infringe on those," District Attorney Tony Rackauckas told reporters.
Investigators said witnesses said Tabares appeared "out of it" as he approached the store at a busy intersection near a high school. Toxicology tests later showed he tested positive for methamphetamine.
Assistant District Attorney Ebrahim Baytieh said Esparza gave multiple commands that Tabares ignored and tried using his Taser before shooting his weapon. A jury, he said, would find "he was doing his job as a peace officer in a reasonable manner."
Brandon Tabares said his brother's life had unraveled into homelessness, drug use and mental illness after he failed a drug test and was discharged from the Navy five years ago.
Dillan Tabares had a series of arrests in California and was released on parole Sept. 14 after completing a 2-year sentence for a felony battery conviction, the district attorney's office said. He absconded and was marked as a "parolee-at-large" on Sept. 20 - two days before the shooting.
His family repeatedly sought help for him and said he was treated at psychiatric facilities.
A message was sent to his brother Wednesday seeking comment.
In November, authorities said they believed that days before the shooting Tabares had beaten to death an 80-year-old friend. It took about a month for police to tie the two cases together.
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