LOS ANGELES >> A grand jury declined to bring charges against an off-duty Los Angeles police officer who believed he had been shot in the head when he opened fire at a Southern California Costco warehouse, killing a mentally ill man who assaulted him, prosecutors said today.

Riverside County District Attorney Mike Hestrin said his office would not bring its own charges against Officer Salvador Sanchez in the wake of the panel’s decision.

Sanchez, a seven-year veteran of the LAPD, opened fire June 14, fatally shooting 32-year-old Kenneth French and critically injuring his parents, Russell and Paola French.

“In a case like this, it’s appropriate for the community to weigh in,” Hestrin said at a news conference. “This case has weighed heavily on us.”

Dale Galipo, an attorney for the French family, said today he was “disappointed but not surprised” by the news. He said he believes prosecutors only brought the case to a grand jury to appease the community and may not have pushed hard enough to convince the jurors.

The encounter in the Corona warehouse store spanned 3.8 seconds, requiring investigators to rely on a single, poor-quality clip of surveillance video and witness testimony — some of which had to be compelled through subpoenas.

Sanchez, holding his 1½-year-old son, was standing in line for food samples with his wife around 7:45 p.m.

French, without warning or provocation, struck him in the back of the head, knocking Sanchez and his child to the ground. Seconds later, prosecutors said, Sanchez fired 10 rounds from his handgun — believing his life and his son’s life were in immediate danger from an active shooter.

Four bullets struck French in the back and shoulder, one struck his mother in the stomach and another hit his father in the back, Corona Police Chief George Johnstone said.

The gunfire prompted chaos inside the warehouse as terrified shoppers rushed to leave while police officers — who also believed there was an active shooter — ran inside.

Witnesses reported seeing Sanchez reach to the back of his head multiple times to look for blood, Hestrin said, noting that Sanchez’s claim of being knocked unconscious was not supported by any evidence.

Galipo has said Russell and Paola French were trying to explain to the officer that his son had a mental disability when shots rang out. 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.

The family has filed a claim — a precursor to a lawsuit — against Sanchez and the Los Angeles Police Department. Galipo said he plans to file a federal lawsuit within the next month.

The LAPD is conducting an administrative inquiry into whether Sanchez followed the department’s policies.

Sanchez’s lawyer could not immediately be reached today.