The 16-year-old boy suspected of killing two students in a burst of gunfire at a school outside Los Angeles has been described as a quiet, smart child you would never have expected to turn violent.

One fellow pupil at Saugus High School said the suspect was a Boy Scout who she relied on for help with history, and a student in his physics class said he seemed like “one of those normal kids”.

A next-door neighbour who grew up with him said he kept to himself but was never threatening.

The attacker shot five pupils, seemingly at random, and then shot himself in the head at about 7.30am Thursday, his 16th birthday, authorities said.

Two students died and the gunman was gravely wounded. Police have not publicly identified the gunman because of his age.

The Associated Press determined his identity based on property records for his home, which police said was searched after the shooting, and interviews with three of his friends.

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.

California High School Shooting
People hug each other during a vigil for the Saugus High School shooting victims (Hans Gutknecht/The Orange County Register/SCNG/AP)

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

“A quiet, to-himself kid,” said Ryan McCracken, a 20-year-old next-door neighbour. “You wouldn’t expect anything like that from him.”

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.

California High School Shooting
Students are escorted away from the school after the shooting (David Crane/The Orange County Register/AP)

They confirmed that a message, “Saugus, have fun at school tomorrow”, was posted to the Instagram account believed to belong to the suspect before the shooting, and were trying to determine its relevance and whether someone else with access to the account might have written it.

Brooke Risley, a fellow pupil at Saugus High, 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 the Boy Scouts.

Joe Fitzpatrick, a senior pupil who helped the teacher in the boy’s physics class, called him a “good, quiet kid” who did not miss assignments and did well in tests.

“He just seemed like one of those regular kids,” Mr Fitzpatrick said.