A grand jury declined to bring charges against an off-duty Los Angeles police officer who fatally shot a mentally ill man who assaulted him in a Southern California Costco warehouse, Riverside County District Attorney Mike Hestrin said Wednesday. The officer was holding his child when he was assaulted.

Officer Salvador Sanchez, a seven-year veteran of the LAPD, opened fire on June 14, fatally shooting Kenneth French and wounding his parents, Russell and Paola French. A family attorney has said Kenneth French, 32, was nonverbal and suffered from schizophrenia.

Hestrin said the results of the investigation were presented to a grand jury this month, and the panel declined to return charges.

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Hestrin says he's not sure why the grand jury arrived at their decision, but said there was no previous encounter or words exchanged before French struck Sanchez.

"It was out of the blue, and the officer wasn't expecting it — he's hit in the back of his head while he's holding his child in his hands," Hestrin said. "I can only imagine those facts played into the jury's decision here."

An undated photo of 32-year-old Kenneth French, and his parents Russell and Paula French, who were shot by an off-duty LAPD officer at a Costco in Corona, Calif., on June 14, 2019. Kenneth died but his parents survived. CBS Los Angeles

Hestrin said the incident happened "very fast," with Sanchez firing less than four seconds after he was struck. The officer believed he had been shot and that he was firing at an active shooter, officials said during a Wednesday press conference, citing witnesses who heard him say he had been shot and saw him reach to the back of his head multiple times to look for blood.

Sanchez reportedly suffered minor injuries, and his son was not hurt.

Hestrin played grainy, low-quality surveillance video of the incident during a Wednesday press conference.

Lawyers for Sanchez have said he acted in self-defense when French attacked him as he was holding his 1½-year-old son during a shopping trip to the warehouse store in Corona. French knocked him to the ground, Sanchez's attorney David Winslow has said.

Dale Galipo, an attorney for the French family, has called it an "open-handed push or slap" to the policeman's back.

Believing his life and his son's life were in "immediate danger," Winslow has said, Sanchez fired his handgun. Ten rounds were fired, officials said Wednesday. Four bullets struck Kenneth French, and each parent was hit once and wounded. The LAPD is conducting an administrative inquiry into whether Sanchez followed the department's policies. The officer is on paid administrative leave, reports CBS Los Angeles.

Galipo has said Russell and Paola French were trying to explain to the officer that their son had a mental disability when Sanchez opened fire. Kenneth French, of Riverside, had been taken off his medication because of other health complications, which may have affected his behavior that night, Galipo has said.

Surveillance video of the physical confrontation and shooting taken on June 14, 2019. Riverside County District Attorney's Office

In a news conference last month, the family claimed they pleaded with Sanchez not to open fire, CBS Los Angeles reported.

"I told Officer Sanchez not to shoot, twice, I even said 'please,' I was pleading for our son and our lives," Paola French said.

The family has filed a claim — a precursor to a lawsuit — against Sanchez and the Los Angeles Police Department.

Hestrin called the shooting a "horrible tragedy" and said the case has "weighed heavily on all of us." He said he could have opted to file a charge against Sanchez despite the grand jury's decision, but decided not to because he felt he couldn't prove a case beyond a reasonable doubt in court.