One of the five students shot during the Saugus High School shooting in California has been identified as her aunt revealed how she managed to survive.
The teenager, named only as Addison, had surgery and is expected to make a full recovery.
Good Morning America reported that Addison stumbled into a room where choir teacher Katie Holt sheltered 40 children behind a piano as gunfire broke out, reports News.com.au.
"Addison rushed in. We cleared a space for her, I wrapped the wound," Ms Holt said. "She had a gunshot wound. I should have never had to treat a gunshot wound as a choir teacher. I held a bleeding child today in a room of 40 children."
"I don't think there's any sort of training that can prepare you for this," she said.
The reported suspect shot five classmates 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 grave condition.
A 14-year-old boy died at a hospital. Earlier, a 16-year-old girl died at the school.
The three surviving victims remain hospitalised.
His .45-caliber handgun was found with no bullets remaining when it was recovered.
LA County Sheriff's Captain Kent Wegener revealed how the suspect left a message on an Instagram account, which read: "Saugus have fun at school tomorrow."
The message was posted Thursday morning (US time) but it's unclear when. They say it was later removed, and they don't know who made the change.
Wegener said it "may have some significance" as they have obtained a search warrant to raid the home of the accused for more evidence.
NBC reports the shooter's girlfriend and mother were being questioned by authorities.
The FBI says there's no reason to believe he was acting on behalf of a group or ideology or that there were any conspirators.
He was a sophomore at Saugus, who ran track and field for the school and competed in the 800-metre and 1600-metre races, according to stats online.
Surveillance footage of the young boy pulling the gun from his backpack, before firing at his classmates and then shooting himself in the head has also been uncovered.
The boy's mother said she was praying for 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 the violent act.
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".
A childhood friend 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.
Heavy reports the boy's father, Mark, died on December 5, 2017, aged 55.
His mother described how "his dad used to make bullets, so I'd assume he had bullets and stuff like that."
The weapon — a .45 semiautomatic pistol that had no remaining bullets — was recovered at the scene, Capt. 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 told KTLA that he had known the suspect since he was a child.
Mr 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.
A White House spokesman said Donald Trump had been briefed on the shooting, but the US President had yet to make a public statement on the tragedy.
HOW THE SHOOTING UNFOLDED
The Los Angeles County Sheriff's Department said on Twitter that deputies responded to the shooting around 7.45am (2.45am AEDT) at Saugus High School in the city of Santa Clarita.
Authorities say off-duty officers whose children went to Saugus High School in Santa Clarita were the first on scene and provided first aid to the victims.
Collyn Nielson with the William S. Hart Union High School District says initial reports indicate teachers and students "knew what to do" and immediately barricaded themselves in classrooms, turned out lights and hid. Nielson says schools hold lockdown drills three times a year.
He said in addition to one sheriff's deputy Saugus High School has nine "campus supervisors" who act as security guards.
There are a dozen security cameras and no metal detectors.
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 person she didn't know gave shelter to her and about 10 other students.
"I just heard a lot of kids crying. We were scared," Ms Rodriguez said.
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 said 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.
Saugus High School and other schools in the area were locked down.
That lockdown was later lifted.
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.
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.
Her friends heard five shots, they were in the gym when they heard the shots.
It was in the main quad on campus.
"Her friends called her right away and said not to go to school and we went to get all of her friends. 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 knows two students who attend the school, one of whom was barricaded in a classroom.
She tweeted 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."