The ex-convict who killed two co-workers and wounded two others at a Fresno, Calif., processing plant this week had been diagnosed with “intermittent Explosive Disorder,” police say. They added he also had dependences on multiple substances including amphetamine, marijuana and alcohol, reports ABC News.

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. Lawrence Jones, who was employed by Valley Protein, was diagnosed with the condition during a mental health evaluation in 2004.

Two of the victims had worked at the plant for less than a week. Another had worked there since June.

Bob Coyle, president and owner of the Valley Protein plant, said Jones always arrived on time and said hello, and "got the job done" — though he mostly kept to himself. Jones' boss and co-workers were puzzled about what set off the ex-convict who had been in and out of prison since 1991 for crimes ranging from armed robbery to car theft — a record his managers were aware of, Coyle said.

They said Jones, who had worked at the plant for 14 months, was a nice, respectful man, and a model employee. Police have not released a motive for the shootings. After emptying his gun, Jones walked outside of the plant and shot himself in the head, dying while receiving medical care at an area hospital.

Salvador Diaz, 32, and Manuel Verdin, 34, were killed during the attack. Arnulfo Conrriguez, 28, was listed in serious condition Thursday at Fresno's Community Regional Medical Center, a hospital spokeswoman said. Fatima Lopez, who witnessed the slayings and was shot as she started to run, was treated and released.

Source: ABC News, Associated Press