The weapon used in last week’s deadly mass shooting at Saugus High School in Santa Clarita was a a so-called “ghost gun,” a firearm without a registration number, officials at the Los Angeles County Sheriff’s Department said Thursday.

Homicide detectives at the department said they are looking for evidence regarding who purchased and assembled the firearm components and are investigating the shooter’s purchase history on electronic devices seized from his home.

Police say Nathan Berhow came to school the morning of Nov. 14, his 16th birthday, and opened fire with the .45-caliber handgun in the Santa Clarita high school’s quad. Five students were shot, two of whom later died, before Berhow turned the weapon on himself. He later died from his injury.

“The report on the firearm indicates that the handgun used in the assault was not manufactured conventionally and may be some form of a “kit gun,” one that is assembled by a consumer — rather than a manufacturer — from pieces bought separately,” wrote sheriff’s Capt. Kent Wegener in a statement. “We have no evidence to indicate who assembled it or bought the components.”

Wegener said detectives are still investigating social media leads and looking for accounts belonging to the subject. With assistance from the Secret Service, deputies are searching the contents of technology devices recovered from his home such as  tablets, computers and cell phones.

Paperwork is also being confiscated from the home for examination “to see if it contains any evidence related to the subject’s state of mind, or historical information about the weapon used,” Wegener added.