Updated

Authorities say the handgun used by a former convict to shoot co-workers execution-style at a Central California chicken processing plant was stolen.

Fresno Police Chief Jerry Dyer said at a news conference on Wednesday that the four-shot Derringer handgun was traced to a residential burglary in Fresno that occurred in June.

It was not clear, however, who stole the gun or how the suspect in the Nov. 6 attack at Valley Protein, Lawrence Jones, acquired it. Dyer said the serial numbers on the gun were filed off, but federal investigators were able to raise the numbers and match them to the stolen gun.

Police say Jones pulled the weapon and shot four co-workers, two fatally, in the middle of his morning shift at the plant. He then took his own life.

Police have not established a motive.