FRESNO, Calif. (AP) — The handgun used by a former convict to shoot co-workers execution-style at a Central California chicken processing plant was stolen in a residential burglary in June, a police chief said Wednesday.

It wasn't clear who stole the four-shot Derringer handgun or how the suspect in the Nov. 6 attack acquired it, Fresno Police Chief Jerry Dyer said at a news conference. The serial numbers on the gun had been filed off, but federal investigators were able to raise the numbers and match them to the stolen gun, he said.

Lawrence Nathaniel Jones shot four co-workers, two fatally, in the middle of his morning shift at Valley Protein, investigators have said. The 42-year-old then took his own life. A motive hasn't been established.

One of the victims, Arnulfo Conrriguez, who was shot in the neck, remains in stable condition at Fresno's Community Regional Medical Center, Dyer said.

A toxicology test revealed traces of marijuana but no other street drugs in Jones' body, Fresno County Coroner David Hadden said last week. A second test is being conducted to reveal if Jones took psychiatric drugs or other substances, he said.

A Fresno County mental health evaluation in 2004 diagnosed Jones with intermittent explosive disorder, drug-induced psychotic disorder and dependence on multiple substances including amphetamine, marijuana and alcohol.

Jones often refused medication and was described by staff as mentally unstable, according to the evaluation.

Intermittent explosive disorder is a condition marked by sudden aggression that appears out of proportion to any external stress, according to the National Institute of Mental Health. To be diagnosed with it, a person must have had three such aggressive episodes.

The shooter's family members told investigators that Jones had violent tendencies, Dyer said.

The police chief also said investigators found two dozen unsent letters in Jones' apartment, hand written to various female newscasters and other women Jones met on the street. In the letters, Jones commented on the women's clothing and gestures, as well as talked about rap music, according to investigators.

The letters did not contain references to violence, threats, or sexual content, Dyer said. But Jones signed them as the deceased Oakland drug lord Felix Mitchell, or as the former president of Nigeria, Olusegun Obasanjo — a further sign, Dyer said, that the shooter was a "deeply disturbed individual."