A 16-year-old boy who killed two students and wounded three others in a high school shooting has died in hospital as investigators continue to search for a motive for the attack.

Nathaniel Tennosuke Berhow, succumbed to a head wound he suffered after turning his gun on himself following Thursday's attack in Southern California.

Los Angeles Sheriff’s Department said​ Berhow’s motive “remains a mystery” and investigators have not found any manifesto, diary or suicide note to explain his actions, despite conducting more than 40 interviews.

Gracie Anne Muehlberger, 15, and Dominic Blackwell, 14, were killed in what Sheriff Alex Villanueva described as a "planned attack".

Two girls and a boy were also injured but are recovering.

Santa Clarita school shooting Show all 20 1 /20 Santa Clarita school shooting Santa Clarita school shooting Students are escorted out of Saugus High School after reports of a shooting AP Santa Clarita school shooting At least six people have been injured following reports of a shooting at the school, roughly 40 miles north of Los Angeles in Santa Clarita KTLA Santa Clarita school shooting Students are comforted as they wait to be reunited with their parents AP Santa Clarita school shooting Paramedics transport an injured person AP Santa Clarita school shooting Students get on a bus after the shooting AFP via Getty Images Santa Clarita school shooting Police search a home in connection with the shooting AP Santa Clarita school shooting Students are escorted out of Saugus High School AP Santa Clarita school shooting D J Hamburger, center in blue, a teacher at Saugus High School, comforts a student AP Santa Clarita school shooting People are lead out of Saugus High School after reports KTTV-TV via AP Santa Clarita school shooting Women embrace in Central Park after the shooting AFP via Getty Images Santa Clarita school shooting People wait outside of Saugus High School after reports of a shooting on Thursday, Nov. 14, 2019, in Santa Clarita, Calif. Los Angeles County Sheriff Alex Villanueva tweeted that the suspect was in custody and was being treated at a hospital. He said the suspect was a student but gave no further information.(AP Photo/Marcio Jose Sanchez) Marcio Jose Sanchez AP Santa Clarita school shooting Students line up after a shooting AFP via Getty Images Santa Clarita school shooting People wait for students and updates outside of Saugus High School AP Santa Clarita school shooting Ambulances parked outside of Saugus High School AP Santa Clarita school shooting Law enforcement personnel gather outside Saugus High School AP Santa Clarita school shooting The sheriff's office says a male suspect in black clothing was seen at the school NBCLA via Reuters Santa Clarita school shooting A law enforcement official leads students at the scene VIA REUTERS Santa Clarita school shooting Police are searching for a 15-year-old suspect KTTV-TV via AP Santa Clarita school shooting A law enforcement official leads students at the scene VIA REUTERS Santa Clarita school shooting Police are searching for a 15-year-old suspect following a shooting at a high school in Santa Clarita, California.

Berhow opened fire at around 7.30am on Thursday, his birthday, after being dropped off at Saugus High School in the Los Angeles suburb of Santa Clarita.

Mr Villanueva said Berhow counted his rounds and fired six shots before using the last bullet on himself in the attack, which last just 16 seconds.

“As far as we know the actual targets were at random,” the sheriff said.

The police’s conclusion that the attack was planned was based on how Berhow brought the weapon, handled it ably and kept track of the rounds fire, Mr Villanueva said.

In a statement, Gracie Anne's parents Bryan and Cindy Muehlberger said they shared the news of their daughter's death with “unexplainable brokenness.”

The couple added: “We miss her smile, laughter, sweet kisses, and her amazing sense of humour.

“We will even miss her constant pestering for Starbucks and Cold Stone and anything else with lots of sugar in it. It must have been the reason why she was so sweet.”

Berhow was described as a quiet and intelligent child, who had not displayed any signs of violence prior to the attack.

Mr Villanueva said investigators have discovered “nothing out of the ordinary” about him, adding: “He's a cookie-cutter kid that you could find anywhere."

Marisa Randazzo, a psychologist who co-wrote federal guidelines for assessing school shooting threats, said the stereotype of shooters as isolated sociopaths was often inaccurate.

The former chief research psychologist for the US Secret Service said most of the shooters she studied were academically successful and were not social outcasts.

She added they were often pushed by some kind of loss or disappointment, often recent, followed by an inability to cope with a feeling of being overwhelmed.

“These are acts of suicide as much as homicide,” Ms Randazzo said.

Police have interviewed Berhow’s friends, who said he could be introverted but had a good social network and a girlfriend.

The boy’s father was an avid hunter who died two years ago and police said they found several firearms at his home, some of which were unregistered.

The Bureau of Alcohol, Tobacco, Firearms and Explosives is working with police to determine where the teenager got the handgun used in the attack.