The weapon used in the deadly Saugus High School shooting in Southern California was a “ghost gun” constructed from various parts to make it difficult to trace, authorities said Thursday.

Detectives are investigating who assembled the unregistered .45-caliber handgun that the alleged shooter, 16-year-old Nathaniel Berhow, fired at classmates Nov. 14 on the campus in Santa Clarita, the Los Angeles Times reported.

The rampage claimed the lives of 16-year-old Gracie Muehlberger and 14-year-old Dominic Blackwell before Berhow turned the gun on himself and died a day later, officials said.

Investigators are looking at Berhow’s electronic communications in an effort to find out more about who built the gun, which was a 1911-model pistol, the newspaper reported.

Berhow’s late father was a gun owner and authorities seized guns, many of which were unregistered, from the teen’s home, the report said.