LOS ANGELES (AP) — The teenager who shot five classmates, killing two, at a Southern California high school used an unregistered “ghost gun,” Los Angeles County Sheriff Alex Villanueva said Thursday.

Villanueva told media outlets Thursday that the 16-year-old shooter’s .45-caliber semi-automatic pistol was assembled from gun parts and did not have a serial number.

The shooter pulled the gun from his backpack on Nov. 14 — his birthday — in an open-air quad at Saugus High School in the Los Angeles suburb of Santa Clarita and in 16 seconds shot five students at random. At one point, the gun jammed, but he quickly cleared it and continued shooting.

He counted his rounds, Villanueva has said, saving the last bullet for himself.

Sheriff’s Lt. Brandon Dean of the homicide bureau said in a statement that authorities do not know who assembled the pistol or bought its components.

The sheriff said the shooter’s motive remains a mystery, even after investigators searched his home and interviewed 45 people. The teen’s mother had no idea what was coming, Villanueva said.

Authorities said he had shown no signs of violence and didn’t appear to be linked to any ideology or terrorist group.

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

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.

Authorities have identified the dead as 15-year-old Gracie Anne Muehlberger and 14-year-old Dominic Blackwell.