SANTA ROSA, California (Reuters) - No more than 10 seconds elapsed from the time sheriff's deputies spotted a 13-year-old California boy carrying what they thought was an assault rifle and the moment they shot him dead, only to learn afterwards the gun was a plastic replica, the police said on Thursday.

It took 16 seconds more for the two officers to call for medical assistance, according to the timeline of events released by police investigating Tuesday's shooting in Santa Rosa, a suburb in northern California's wine country.

Andy Lopez Cruz, wearing a hooded sweatshirt, had been on his way to a friend's house clutching the imitation gun designed to shoot plastic pellets, the police said. He died at the scene. A toy handgun also was found tucked in his pants.

An autopsy performed on Thursday found seven bullets struck the boy, and that two of the wounds were fatal. Investigators believe a total of eight rounds were fired by one of the two deputies who confronted the boy.

The officers involved in the shooting, two deputies of the Sonoma County Sheriff's Department, have been placed on administrative leave, and the incident was under investigation by the Santa Rosa Police Department and other law enforcement agencies.

But the tragedy has reignited calls in the community for creation of civilian review boards to examine such incidents.

"People have to do something," said Mr Elbert Howard, a founding member of the Police Accountability Clinic and Helpline of Sonoma County. "He's a child, and he had a toy. I see that as an overreaction to shoot him down."

An advisory panel of the United States Civil Rights Commission urged Sonoma County to create civilian-review boards in 2000 following eight fatal officer-involved shootings in less than three years, but that recommendation went unheeded.

As many as 200 mourners gathered on Thursday around a makeshift memorial consisting of flowers, balloons, teddy bears and pictures of the boy at the site of the shooting.

Some held candles and signs that said: "What a tragedy, what a travesty."

Friends and family have described the boy as a well-liked eighth-grader who played the trumpet and basketball and had a good sense of humour.

Police Lieutenant Paul Henry issued a brief chronology of the killing that showed the shooting unfolded swiftly following an alert about a suspicious person in the area.

Four seconds later, the two officers radioed for assistance from other deputies. Then six seconds more passed before they notified dispatchers that shots had been fired. Police have previously said the deputies called for backup as soon as they noticed what appeared to be the military-style rifle.

According to that account, one of the deputies opened fire after the boy was twice ordered to put the gun down and instead turned toward the officers as the barrel of the weapon rose in their direction.

Police also have said the deputy who opened fire could not tell from the angle of his position that he was shooting at a child, but saw the gun and feared for his life.

The sheriff's department has refused to identify the deputies involved. Their lawyer has declined to comment.

"We want justice," the boy's father Rodrigo Lopez said during a silent vigil in front of Santa Rosa City Hall on Wednesday.

"I don't want the same thing to happen to other families."