The firearm used in the deadly Riverside, California, shootout was an untraceable AR-15-type weapon assembled from separately acquired parts, commonly known as a "ghost gun," a law enforcement source told CNN on Thursday.
"Ghost guns" are firearms manufactured without serial numbers, making them difficult for law enforcement authorities to trace, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Gun parts can be ordered online and assembled at home, the ATF said.
Riverside Police only describe the firearm as "a possible semi-automatic rifle," spokesman Ryan Railsback told CNN. "It has not been sent out to the crime lab yet for evidence collection or ballistics testing, and has not been evaluated by the California Department of Justice or ATF for retracing (its) history."
Authorities say Aaron Luther killed California Highway Patrol Officer Andre Moye and wounded two others in Monday's shootout in Riverside. Luther, an ex-convict, was shot during the exchange of fire and was pronounced dead at a hospital.
Moye was shot after pulling over a vehicle near I-215 and Eastridge Avenue. He managed to put out a call for help, which brought officers from CHP, Riverside Police and the Riverside County Sheriff's Department to the scene, who exchanged gunfire with the suspect.
Moye served as a CHP officer for nearly three years and was assigned to Riverside after graduating from the CHP Academy in 2017, a release from California Gov. Gavin Newsom's office said. Moye is survived by his wife, parents, and siblings, the release said.
An increase in the number of ghost guns
It is not known whether any parts of Luther's gun had specific manufacturer markings, according to CNN's source.
Los Angeles has seen an increase in the number of ghost guns, the LAPD said in a statement released in 2018. Detectives often recover ghost guns, particularly in the form of semi-automatic 9mm pistols, at various scenes of criminal activity, according to the LAPD.
Ghost guns are "particularly useful to individuals who are banned by police from owning traditionally purchased guns," allowing owners to sidestep the legal system requiring gun registration, according to a Utah State University paper published earlier this year.
It's not known how many ghost guns are in the US today, though one regional ATF office in California obtained 250 ghost guns in 2017 alone, the paper noted.