A US teen who allegedly killed two students when he opened fire at his Los Angeles high school on his 16th birthday left an eerie message on social media before his violent act.

Nathan Berhow, 16, is the reported suspect who killed a 16-year-old-girl and 14-year-old boy after he pulled a gun from his backpack and began his 16-second attack at Saugus High School in the city of Santa Clarita.

He then turned the .45 calibre semi automatic pistol on himself and is now in hospital in a "critical condition", reports news.com.au.

Emergency personnel remove an injured person following a shooting at Saugus High School. Photo / AP
Emergency personnel remove an injured person following a shooting at Saugus High School. Photo / AP

One teenager has also been discharged after receiving medical treatment.

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LA County Sheriff's Captain Kent Wegener revealed he left a message on an Instagram account, which read: "Saugus have fun at school tomorrow."

He said it "may have some significance" as they have obtained a search warrant to raid his home for more evidence.

Berhow is a half-Japanese, half-American student whose family lives in Valencia, California, about 3.2km from the school.

Nathan Berhow, who is in hospital in a critical condition, is the key suspect in the Saugus High shooting. Photo / Facebook
Nathan Berhow, who is in hospital in a critical condition, is the key suspect in the Saugus High shooting. Photo / Facebook

He is a sophomore at Saugus, and ran track and field for the school and competed in the 800-metre and 1600 metre races, according to stats on Athletic.net.

Surveillance footage of the boy pulling the gun from his backpack, before firing at his classmates and then shooting himself in the head has also been uncovered.

Berhow's mother, Mami Matasuura, said she was praying her son.

"I don't even know what to do right now, I am just praying for my son's life," she said.

Students spoke of their terror and shock after the shooting, revealing some possible motives for Berhow's violent act.

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LA Police have refused to identify the boy officially as the FBI joins their probe into why he carried out the fatal shooting.

His peers have described him as 'unsociable'.

Police vehicles fill the street in front of the home of the alleged shooter in this image from KTLA-TV. Photo / AP
Police vehicles fill the street in front of the home of the alleged shooter in this image from KTLA-TV. Photo / AP

A childhood friend who only identified himself by his first name of Ryan to KTLA, said he was quiet and introverted and had lost his father.

He said the suspect's father died about a year ago and speculated his death may have been linked to the shooting.

"We were playing the backyard... over the years we stopped talking, there was a disconnect (between us)," he said.

"He was doing his own thing. It's a shock to all of us. His father passed a year ago or something like that."

A parent waits outside following the shooting. Photo / AP
A parent waits outside following the shooting. Photo / AP

"His dad used to make bullets, so I'd assume he had bullets and stuff like that."

The weapon β€” a .45 semiautomatic pistol that had no remaining bullets β€” was recovered at the scene, Captain Kent Wegener said.

"Detectives have reviewed video that clearly shows the suspect withdraw a handgun and wound five people and shoot himself in the head," he said.

A next-door neighbour, Jared Axen, told KTLA that he had known the suspect since he was a child.

Medical personnel load an injured person into an ambulance outside Saugus High School. Photo / AP
Medical personnel load an injured person into an ambulance outside Saugus High School. Photo / AP

Axen said his father used to go hunting, so firearms "weren't a mystery to him".

"I can't think of any red flags in our conversation, when he would think of handling his hurt in this manner," he said.

"He grew up with his father who was a hunter, and this was a normal activity for him.

"He grew up around firearms, it wasn't a mystery, he knew how to use it and understood how it worked, he was responsible. He came off as a very responsible person."

Authorities are working to obtain any other video from campus and mobile phone video from students.

Students are escorted off of campus after the shooting. Photo / AP
Students are escorted off of campus after the shooting. Photo / AP

A White House spokesman said Donald Trump had been briefed on the shooting.

"We continue to monitor the terrible events at Saugus High School in Santa Clarita, California, through our ongoing communications with Local, State and Federal Authorities. We send our deepest condolences to the families and friends of those tragically lost and we pray for the speedy recovery of the wounded," Trump tweeted.

How the shooting unfolded

The Los Angeles County Sheriff's Department said on Twitter that deputies responded to the shooting around 7:45am (4:45am NZT) at Saugus High School in the city of Santa Clarita.

Student Rosie Rodriguez said she was walking up the library stairs when she heard noises that "sounded like balloons" popping. She realised 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 stranger sheltered her and about 10 other students.

"I just heard a lot of kids crying. We were scared," Rodriguez said.

Students wait to reunite with their families at Central Park following the shooting. Photo / AP
Students wait to reunite with their families at Central Park following the shooting. Photo / AP

On a normal day, she said, Saugus High School felt very safe. "We never really thought this would happen in our school," she said.

Henry Mayo Newhall Hospital tweeted that one female patient was dead, two males were in critical condition and one male was in good condition. It was unclear if there were any others injured. Initial reports fluctuated as high as six.

The sheriff's office said the suspect was in custody and being treated, an hour and a half after reports of the shooting first surfaced.

He was reported to be a 15-year-old student from the school, of Asian appearance, and wearing dark clothing.

Another student said she initially mistook gunshots for instruments in a band class.

Shauna Orandi was in her Spanish class working on homework when she heard gunfire.

She says she thought her worst nightmare had come true. She said to herself: "This is it. I'm gonna die."

She was later escorted from the school and reunited with her father in a nearby park.

A student is escorted out of Saugus High School after the shooting. Photo / AP
A student is escorted out of Saugus High School after the shooting. Photo / AP

Saugus High School and other schools in the area were locked down.

Television images showed police swarming the school and several people being moved on ambulance stretchers. Lines of students were escorted from the school by armed police.

Earlier frightened students barricaded themselves and hid inside their classrooms while worried parents frantically rushed to campus.

"They're texting us that they're hiding in closets, they're scared to die," one mother told KTLA.

Another parent told KTLA she received a text message from her daughter.

"She told me, 'Mom, I'm so scared. I love you guys so much. Please come, please come'," the woman told KTLA. "That just killed me."

Cari Wright received a devastating text from her daughter.

"Hey mom I don't know what's going on here at school but I love you and I'm so thankful for everything you've done for me. I love you so much," the student sent her mother, who shared the texts with ABC7.

"Everyone is saying there's a shooter on campus. I don't know what's going on but I love you and dad so much."

"I love you baby," Cari Wright responded to her daughter. "Stay safe."

Keegan Burns, 16, reunites with her parents Jim and Mirasol in Central Park. Photo / AP
Keegan Burns, 16, reunites with her parents Jim and Mirasol in Central Park. Photo / AP

Anxious parents were awaiting reunification with their children in a park near the high school.

Los Angeles County Undersheriff Tim Murakami has tweeted an apology to the parents, saying investigators needed to interview the students before they could be released.

Student Sharon Orelana Cordova told KNBC-TV she hid under a table in a nurse's office until officers came to get her.

"When I got out, I saw this person lying on the ground … with blood all over," she said.

Student Mason Peters told KCAL-TV his class heard a "distinctive sound" outside. "And so my teacher quickly sprang to his feet, got up, locked the door, asking the students to get the keys," he said.

"And so we like reinforced it, turned off all the lights and then we got a bunch of desks and stuff and reinforced the doors. And we all just like stayed hidden."

Hannah de Caussin was on her way to school with her child when her daughter received a call from friends at the school, CNN reports.

A student is embraced at a park following the shooting. Photo / AP
A student is embraced at a park following the shooting. Photo / AP

"We were just about to leave to go to school," de Caussin said.

"We are all really upset. This is a peaceful neighbourhood and this is shocking. The kids are texting all of their friends and trying to figure out who the victims are."

America reacts to the shooting

Actress Kristy Swanson, who starred in Buffy the Vampire Slayer, tweeted that she knew two students who attend the school, one of whom was barricaded in a classroom.

She said her young friend Madi was "barricaded in a classroom" but "OK".


Ellen DeGeneres tweeted that "my heart is with the community of Santa Clarita and Saugus High School", while US talk show host Ryan Seacrest implored Americans to not "grow numb to this violence".

Democratic politicians also took to Twitter to condemn the latest shooting, which has become an indictment on the ruling party's inability to make serious gun reform changes.

Democratic Presidential nominee frontrunner Elizabeth Warren said, "We shouldn't have to live like this. Students shouldn't live in fear when they go to school."


Speaker of the House, Nancy Pelosi, had a crack at Republicans, tweeting, "What will it take for Republicans to stop blocking efforts to #EndGunViolence?"

Kamala Harris said, "Our children and communities are being terrorised. We can't accept this."

Bernie Sanders said: "We have a moral obligation to say: children's lives are more important than gun manufacturers' profits. We must pass common sense gun safety legislation."

-with AP