Workplace violence is as common as water-cooler gossip, with nearly two million incidents - including 600 to 800 homicides - reported nationally.

But count only incidents with more than one dead victim and the numbers shrink to just 10 to 20 cases a year, said Park Dietz, a workplace-violence expert and forensic psychiatrist.

And cases with a female shooter?

"That's so unusual. This is just the fourth one that I recall in 30 years," said Dietz, founder and president of the Threat Assessment Group, of Newport Beach, Calif.

One of the others also occurred here: Sylvia Seegrist went on a shooting rampage at the Springfield Mall in Delaware County in 1985, killing three and wounding seven. She was found guilty of murder but mentally ill and received three life sentences.

Dietz said that typically, workplace shooters erupt after public humiliation, such as an escorted ouster from the workplace, he added. Yvonne Hiller, 43, the suspect in Thursday night's shooting at the Kraft plant, had been suspended minutes before she returned with a .357 Magnum, police said.

And most workplace shooters, Dietz said, have the common denominator of "paranoid perceptions, distortion and depression."

Hiller filed a complaint last year with the U.S. Occupational Safety and Health Administration claiming that co-workers had been spraying her with chemicals, including deer urine.

OSHA ruled after an investigation by Kraft that the allegations were unfounded.