Police seek motive in California school shooting that killed 2, left suspect in 'grave condition' The early-morning attack was on the suspect's birthday.

At least two students are dead and three others injured after a classmate opened fire at a high school in Southern California Thursday morning, sheriff's officials said.

The 16-year-old male suspect was taken into custody and was last reported to be at a local hospital in "grave condition" from a self-inflicted gunshot wound, authorities said.

Detectives reviewed video from the scene which showed the gunman in the quad of Saugus High School in Santa Clarita when he took a gun from his backpack, shot five people and then shot himself in the head, authorities said, noting that the shooting lasted for about 16 seconds.

"From the time that he withdrew the handgun from his backpack to the time that he was on the ground with a gunshot wound to his head was about 16 seconds," Los Angeles County Sheriff's Department Captain Kent Wegener told reporters late Thursday. "We have not yet established a motive or a nexus between the subject and his victims other than to say that they were all students at the high school together."

Authorities served a search warrant at the suspect's purported residence, Wegener said. He said there is no history at that address to indicate there were any issues at the home prior to the shooting. Authorities said there was no evidence to suggest the suspect acted on behalf of a group or with any co-conspirators.

The early-morning school shooting was on the suspect's birthday, authorities said.

The surviving victims are two girls and one boy, officials said. A 16-year-old girl and a 14-year-old boy died after arriving at the hospital.

Terrified students barricaded in classrooms before they fled the campus to search for their concerned parents, who had gathered in the streets.

"I just started running," sophomore Brooklyn Moreno said. "There was girls falling in front of me and I tried to help them up, then just kept running 'cause I didn't want to get hurt, either."

The weapon, a .45-caliber semiautomatic pistol, was recovered with no bullets left, authorities said. Police said the suspect mentioned the school in a message on his Instagram bio that read: "Saugus have fun at school tomorrow." That post has since been removed.

"I can confirm that that was posted to his account, and I can also confirm that it has been changed since this incident, which means there is somebody else that has access to this account, be it a hacker be it a friend that has his password," Wegener told reporters. "That was posted at one point. We are aware of it and we are researching the source, when it was posted and when it was taken down."

Authorities became aware of the post after the shooting.

The suspect's girlfriend and his mother are speaking with detectives, authorities said.

The shooting was reported at 7:38 a.m. local time during what's called "Zero Period," before the school day officially begins at 8 a.m., often a time used for extracurricular classes.

When 17-year-old Hayden Trowbridge heard the gunshots, his classmates pushed their desks to the door as a barricade, he told ABC News.

The teen said he grabbed his metal water bottle to use as a weapon as they all hid under their desks, crying and holding each other.

Trowbridge said during shooting drills he had practiced putting a book in front of his chest to protect against a fatal shot, but he didn’t have anything big enough nearby.

Choir teacher Kaitlin Holt said one girl, shot in the hip and shoulder, was rushed into her classroom by other students. Holt told ABC News she gave the girl first aid and called 911.

Moreno said she was in the school's quad when she heard what she thought was a balloon popping. She took off running.

"I never thought this would happen at my school," Moreno told ABC Los Angeles station KABC. "I'm still kinda in shock right now. I’ve been shaking and crying a lot -- I’m an emotional wreck."

Jeff Turner, a father of three Saugus High School students, said his daughter "was beside herself, hysterical.”

She "thought she should have stayed and helped the kids she saw shot, feeling terrified and guilty all at once," Turner told ABC News. "I broke down in tears. Had to pull myself out of it to help her."

As the search for the suspect was unfolding earlier on Thursday, other schools were placed on lockdown, and officials urged those who live in the area to lock their doors.

Misty Wolf, a Saugus High School graduate whose 16-year-old daughter goes to a nearby school, said they were just arriving when her daughter's school was placed on lockdown.

"We were all getting there and parents heard shots -- or what we thought were shots," Wolf told ABC News. "The nice guy who waves us in the lot every morning started shouting at the kids walking to get out of the way get up the hill. We were all trying to get out. People were confused."

"Having my kid, who is already dealing with things in life -- being scared because I told her to duck down because they don't know where the shooter is -- is hard," Wolf said. "There was another [school lockdown] a few years ago and she never wanted to leave her classroom after it. I'm worried that this will make her not want to be at school because she doesn't feel safe."

Moments before news of the shooting broke, Sen. Chris Murphy, D-Conn., was on the Senate floor calling on his colleagues to bring up a universal background checks bill that was passed by the House earlier this year.

He asked for unanimous consent to pass the legislation dubbed the "Bipartisan Background Checks Act of 2019" that would establish new background check requirements for firearm transfers between private parties. The House approved of the measure in February in a 240-90 vote.

His fellow Democratic Connecticut senator, Richard Blumenthal, was in the middle of his remarks on gun violence when he was handed a note informing him of the reported shooting.

In August, the William S. Hart Union High School District, which includes Saugus High School, voted to extend a contract for school safety for another year for $1.05 million, local newspaper, The Santa Clarita Valley Signal, reported. Schools in the district began holding lockdown drills several years ago, a public information officer for the school district told Santa Clarita radio station KHTS in 2018 after the Parkland shooting in Florida.

ABC News' Olivia Rubin, Matt Gutman, Mariam Khan, Ella Torres, Jesse Convertino, Kelly McCarthy and Julia Jacobo contributed to this report.