Two students have been killed and three others injured by a gunman who opened fire at a secondary school in California, officials say.
The shooting took place at the Saugus High School in Santa Clarita, north of Los Angeles, minutes before the school day was due to begin.
A 16-year-old male suspect, who was also injured, was taken into custody.
Many US schools have implemented active shooter drills in recent years after fatal attacks around the country.
The shooting was first reported at 07:38 local time (15:38 GMT), LA county sheriff Alex Villanueva told a press conference. "Within seconds we received multiple [emergency] calls," he said, adding that the first police units had arrived at the scene within two minutes.
Police found six people suffering from gunshot wounds and transferred them to local hospitals - and the suspect was later identified as one of those injured, Mr Villanueva said.
Two victims - a 16-year-old girl and a 14-year-old boy - died of their injuries, police said. The other victims were two girls aged 15 and 14, and a 14-year-old boy.
The suspect was "identified as a 16-year-old male whose birthday was today", firearms expert Captain Kent Wegener said. He was named by US media as Nathaniel Berhow.
The Associated Press quoted a fellow student, Brooke Risley, as saying he was introverted but "naturally smart", adding that he had a girlfriend and was a boy scout.
According to surveillance footage, the suspect took a gun out of his backpack and shot the five students, before shooting himself in the head, Mr Wegener said.
The weapon recovered was an emptied .45 calibre semi-automatic pistol, he added. Police said they had located the suspect's home and would search it for evidence.
News of the attack emerged during a Senate debate on gun control legislation. Senator Richard Blumenthal, a Democrat, was arguing for gun control when he was given a note with the news.
"We are complicit if we fail to act," he said. "It is not just a political responsibility, it is a moral imperative."
Meanwhile, the acting Homeland Security Secretary, Chad Wolf, said in a statement that his department took school shootings "very seriously" and would help the authorities "develop trainings and resources to improve response capabilities and better protect soft targets".
Gun control, and the right to bear arms, is a divisive political issue in the US. About 40% of Americans say they own a gun or live in a household with one, according to a 2017 survey, and the rate of murder or manslaughter by firearm in the country is the highest in the developed world.
According to the Washington Post, more than 230,000 young people in the US have experienced gun violence at school since the Columbine High School massacre in Colorado in 1999.