The chilling scene played out even as lawmakers in Washington were debating the latest attempt at gun control legislation that has stalled in Congress for months.

Terrified students described barricading themselves into classrooms while others fled as word rapidly spread of the unidentified Asian boy opening fire with a .45 caliber semiautomatic.

"Detectives reviewed the video at the scene, which clearly show the subject in the quad withdraw a handgun from his backpack, shoot and wound five people, and then shoot himself in the head," homicide captain Kent Wegener told a news conference.

The teen was found among the wounded by first responders, said Los Angeles County Sheriff Alex Villanueva.

No motive has yet been established, but a message reading "Saugus, have fun at school tomorrow" had been posted on "what we believe to be the subject's Instagram account," Wegener confirmed.

It was removed by another person following the incident, he added.

The FBI said there was no sign of an ideological motive, or any co-conspirators.

The dead were a 16-year-old girl and a 14-year-old boy. The other three wounded -- one boy and two girls -- were aged between 14 and 15.

Officers went to his home and his mother and girlfriend were at a local police station speaking with detectives.

'My heart sank'

Seventeen-year-old student Andrei Mojica was in his government class when someone burst in and said there was a shooter on campus.

"We had no clue whether the shooter was on the opposite side of campus or right outside our door," he told the newspaper. "That fear made it feel like we were waiting in silence forever."

Finally they were escorted from the classroom by a SWAT team.

A tearful mother waiting beside a nearby church with other parents said she had dropped off her ninth-grader about a minute before the shooting started.

"I haven't seen my son yet but I've talked to him. He's okay ... He just sounded numb," she told an NBC affiliate.

Twenty-four ambulances, seven paramedic units and a helicopter were at the school campus, opposite a residential area, along with dozens of squad cars.

Agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as firefighters, were among those responding.

Gun debate

The incident is the latest of more than 40 shootings at US schools since as recently as May 2018, when a 17-year-old killed 10 people, eight of them students, in Santa Fe, Texas.

Authorities said his shotgun and revolver were legally owned by his father.

Three months earlier a 19-year-old expelled from Marjory Stoneman Douglas High School in Parkland, Florida returned and opened fire, killing 14 students and three staff members.

There is roughly one school shooting a week in the United States, according to Everytown for Gun Safety.

President Donald Trump tweeted that his staff "continue to monitor the terrible events," adding his "deepest condolences to the families and friends of those tragically lost."

The perennial hot-button issue of arms control threatens to spill into next year's election.

"Every day, our children and our communities are forced to endure horrendous acts of violence and bloodshed, while Republicans continue to obstruct meaningful, bipartisan solutions to end the epidemic of gun violence," said Nancy Pelosi, the Democratic speaker of the House of Representatives.

News of the shooting broke during a debate on gun control legislation at the Senate in Washington.

Democrat Senator Richard Blumenthal was speaking as he was handed a piece of paper by an aide.

"On the floor right now there's a school shooting in Santa Clarita, California," he told the chamber.

"How can we turn the other way? How can we refuse to see that shooting in real time, demanding our attention, requiring our action? We are complicit if we fail to act."

California senator Kamala Harris, a Democratic candidate for the 2020 election, said she was "heartbroken and praying for Santa Clarita." —AFP