The teenager who shot and killed several classmates at a southern California high school last week used an unregistered “ghost gun”.

Sixteen-year-old Nathaniel Tennosuke Berhow assembled a .45 caliber semi-automatic pistol from gun parts and did not have a serial number, the Los Angeles county sheriff, Alex Villanueva, said on Thursday. Authorities do not know who assembled the pistol or bought its components.

Berhow pulled the gun from his backpack on 14 November – his birthday – in an open-air quad at Saugus high school in the Los Angeles suburb of Santa Clarita. In just 16 seconds, he shot five students at random. Fifteen-year-old Gracie Anne Muehlberger and 14-year-old Dominic Blackwell later succumbed to their injuries. Berhow died in the hospital the following days.

The sheriff said Berhow’s motive remains a mystery, even after investigators searched his home and interviewed 45 people. Berhow’s mother, who packed him corndogs, grapes and homemade cookies for lunch that morning, had no idea what was coming, Villanueva said.

Authorities said Berhow had shown no signs of violence and did not appear to be linked to any ideology or terrorist group.

The sheriff’s department is working with federal authorities to unlock Berhow’s cellphone, Villanueva said.

Berhow’s father was an avid hunter who died two years ago. He had six registered guns in the family’s home but they were seized in 2016.

Officials found several unregistered firearms in the home after the shooting and are working to determine where those and the weapon used at the high school came from.