A student shot his classmates at his high school stadium in northern Los Angeles on Thursday, killing two of his comrades and wounding three others, before attempting suicide.
US police have begun investigating the incident on Tuesday to find out why a 16-year-old student was shot at his schoolmates that killed two of them.
The shooter died of wounds sustained by shooting himself.
"We know the attack was deliberate, but we don't know the details or the motive at this stage," the Los Angeles police chief told a news conference.
"As far as we know, he (the shooter) chose his targets at random. We were unable to identify any relationship between the suspect and the victims." The young man, Nathaniel Berhao, shot his head after he attacked his comrades on his 16th birthday.
He was taken to hospital and was still in "critical condition" yesterday, before his death was announced. He left no message to explain his actions, and there was no indication that he had ideological motives.
As for the psychological motives, the shooter did not appear to be a victim of bullying at the school, according to the Los Angeles Police Chief.
"He had friends. There were people coming looking for him or taking him to his house," said one of the shooter's neighbors. "I could see him doing a lot of sports."
The gunman had no precedents, but his father, who divorced his mother a few years ago, had alcohol-related problems and was suspected to have been abusing his wife once, but never tried for lack of evidence.