At least two students were killed and four injured when a shooter opened fire early on Thursday morning at Saugus high school in Santa Clarita, 30 miles north of Los Angeles.
The first reports of the shooting in the southern California city came as students were starting the school day. Law enforcement responded in large numbers, scrambling to locate the shooter.
The victims are a 16-year-old female and a 14-year-old male, authorities said on Thursday. Their names have not been released.
The suspect was identified as a 16-year-old Asian male student, currently hospitalized with a self-inflicted gunshot wound and in "grave condition". Local authorities said they were working with the FBI to investigate surveillance footage and social media.
A total of six victims, including the suspect, were found at the scene after the shooting began at 7.38am, before classes began, authorities said. Victims ranging from age 14 to 16 are currently being treated at Henry Mayo Newhall and Holy Cross hospital in Santa Clarita. Several remain in critical condition.
"This is a very tragic day for our community," the local assembly member Christy Smith told reporters at a press conference, thanking law enforcement who responded to the incident within minutes. "We are very close knit. We will come together and survive but we can't afford another day like this."
Speaking to reporters, the Santa Clarita mayor, Marsha McClean, said her own granddaughter has been at the school. "There are no words … The feeling that came over me was indescribable."
Local authorities have said they are working with the FBI to investigate surveillance footage and social media.
Saugus and other schools in the area, including two elementary schools identified by authorities as Rosedell and Highland, were locked down throughout the morning.
Television images showed sheriff's deputies swarming the school and several people being moved on gurneys. Lines of students were escorted away by armed deputies.
Later, scores of parents converged in a park near the high school to reunite with their children.
A student, Sharon Orelana Cordova, told the local station KNBC-TV that she had hidden under a table in a nurse's office until officers had come to get her.
"When I got out I saw this person lying on the ground ... with blood all over," she said.
Rosie Rodriguez, a freshman, was walking up the library stairs when she heard noises that "sounded like balloons" popping. She realized they were gunshots when she saw other students running.
Still carrying a backpack laden with books, she ran across the street to a home, where a person she didn't know gave shelter to her and about 10 other students.
During the more than an hour they were there, students including Rodriguez called their families and texted with friends. Nobody in the house saw what happened directly.
"I just heard a lot of kids crying. We were scared," Rodriguez said.
The sheriff's department said that it believed there was only one suspect but was following all leads.
The White House spokesman Judd Deere said Donald Trump was monitoring the reports. California's governor, Gavin Newsom, said he, too, was monitoring the developments and his administration was working with local law enforcement.
Condolences poured in from across the country, with many Democratic lawmakers pointing at the tragedy to once again argue US lawmakers should pass stricter gun control legislation.
The California senator and presidential candidate Kamala Harris said she was heartbroken and was praying for Santa Clarita. "Our children and communities are being terrorized. We can't accept this," she said.
"Another sad example of a school shooting & why students across the country live in fear of gun violence," wrote the Minnesota senator Amy Klobuchar, also a presidential hopeful.
"I'm heartsick for the victims of this horrifying shooting and their families," said the Massachusetts senator and presidential candidate Elizabeth Warren.