The attacker shot five students, seemingly at random, at Saugus High School and then shot himself in the head about 7.30am on Thursday, his 16th birthday.

A 16-year-old girl and a 14-year-old boy died and two other girls, aged 14 and 15, were wounded, as was a 14-year old boy.

Captain Kent Wegener of the Los Angeles County Sheriff's Department told reporters the entire incident, captured on videotape, took 16 seconds as the young man stood in one spot and fired on one student after another.

"From right where he was standing, he doesn't chase anybody, he fires from where he is until he shoots himself," Wegener said.

Police have not publicly identified the shooter because his is a minor.

The boy lived with his mother in a modest home on a leafy street in Santa Clarita, a Los Angeles suburb of about 210,000 people known for good schools, safe streets and relatively affordable housing.

His father died two years ago. Two years before that, the father had been arrested amid a domestic dispute with the boy's mother.

Brooke Risley, a Saugus High student, said she had known the boy since elementary school and sometimes studied with him on engineering projects.

Risley described the boy as somewhat introverted, though open with his close friends, and "naturally smart".

She said he was not bullied, had a girlfriend and had been an active member of a local Boy Scout troop.

"A quiet, to-himself kid," said Ryan McCracken, a 20-year-old next-door neighbour. When McCracken was younger, he said they played together, sometimes in boy's backyard tree house.

"You wouldn't expect anything like that from him."

Joe Fitzpatrick, a senior who helped in the boy's physics class, called him a "good, quiet kid" who didn't miss assignments and did well on tests.

"He just seemed like one of those regular kids," Fitzpatrick said.

Police said they had yet to determine a motive and any relationship between the gunman and the victims.

Authorities said they have no indication the boy was acting on behalf of any group or ideology.

Wegener said the suspect posted a message on an Instagram account before the shooting that said: "Saugus have fun at school tomorrow."

However, a spokeswoman for Instagram's parent company Facebook told Reuters the account did not belong to the suspect. The post was taken down and the account removed.

According to Los Angeles County Sheriff's Department record, his father was arrested in June 2015 on suspicion of attempted battery on his wife. It's unclear how the case was resolved.

The shooting was the 85th incident of gunfire at a US school in 2019, according to Everytown, a gun control advocacy group.

Australian Associated Press