A candlelight vigil is planned on Sunday evening to remember two students shot and killed by a classmate at his Southern California high school.

It comes as investigators try to determine what prompted the deadly attack that left three other teenagers wounded.

Saugus High School will remain closed until December 2 but counsellors will be available to help students cope with their grief, school district officials in the Los Angeles suburb of Santa Clarita said.

California High School Shooting
Flowers placed in front of Saugus High School in the aftermath of the shooting (Marcio Jose Sanchez/AP)

Detectives were searching for a motive for the killings carried out by Nathaniel Tennosuke Berhow on Thursday, his 16th birthday.

The teenager pulled a .45-caliber handgun from his backpack and shot five students at random before shooting himself in the head. He died the next day.

The dead were identified as 15-year-old Gracie Anne Muehlberger and 14-year-old Dominic Blackwell.

They will be honoured at a 7pm vigil at Santa Clarita’s Central Park.

A wounded 14-year-old girl was released from the hospital late on Friday.

A 15-year-old girl remained in hospital in good condition. A 14-year-old boy was treated and released on Thursday.

Berhow had shown no signs of violence and did not appear to be linked to any ideology, authorities said.

After more than 40 interviews and a search of his home, authorities still were in the dark, Captain Kent Wegener of the Sheriff’s Department’s homicide unit said at a press conference Friday.

“We did not find any manifesto, any diary that spelled it out, any suicide note or any writings,” he said.

Meanwhile, authorities were investigating online threats against schools in and around the Santa Clarita, sheriff’s officials said on Sunday.

Investigators have not found any of the threats made on social media to be credible, officials said.

Additional law enforcement will be posted at area schools on Monday.