(CNN) 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.

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