A 16-year-old boy planned the attack that killed two students and wounded three others at a Southern California high school, but investigators were so far unable to find out why he brought a gun to campus and opened fire, authorities say.
After more than 40 interviews and evaluation of evidence, no motive had been established, said Captain Kent Wegener, of the Los Angeles County Sheriff's Department's homicide unit.
He said no manifesto, diary or suicide note had been found.
"It still remains a mystery why," Sheriff Alex Villanueva told a press conference.
The teenager opened fire on his birthday on Thursday morning after being dropped off by his mother at Saugus High School in the Los Angeles suburb of Santa Clarita.
Nathaniel Tennosuke Berhow fired the final bullet into his own head.
He died at 3.32 pm on Friday at a hospital with his mother present, according to a Sheriff's Department statement.
The shooting in an outdoor plaza took just 16 seconds and was recorded on security video, authorities said.
The teen stood by himself, did not appear to interact with anyone and then walked to the centre of the quad, Mr Villanueva said.
"As far as we know the actual targets were at random," the sheriff said.
"It wasn't a spur-of-the-moment act."
The origin of the gun was being investigated.
Three off-duty law enforcement officers were first on the scene and treated some of the wounded until paramedics arrived.
The dead were identified as 15-year-old Gracie Anne Muehlberger and 14-year-old Dominic Blackwell.
Students are escorted out of Saugus High School. Source: AP
The names of the wounded students were not released.
Doctors said on Friday morning that two girls, ages 14 and 15, who were both shot in the torso, were doing well and should be released from the hospital over the weekend.
A 14-year-old boy was treated and released from another hospital, authorities said.
