LOS ANGELES (AP) – Authorities say the teenager who shot five classmates, killing two, at a Santa Clarita high school about 80 miles south of Bakersfield used an unregistered “ghost gun.”

Los Angeles County Sheriff Alex Villanueva told media outlets Thursday that Nathaniel Tennosuke Berhow’s .45 caliber semi-automatic pistol was assembled from gun parts and did not have a serial number.

Authorities are trying to determine how Berhow got the gun.

Berhow died from a self-inflicted gunshot wound after the Nov. 14 shooting at Saugus High School in the Los Angeles suburb of Santa Clarita.

Authorities say Berhow had shown no signs of violence and didn’t appear to be linked to any ideology or terrorist group . Villanueva says Thursday his motive remains a mystery.

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