NEW WINDSOR-A fired worker who shot three factory managers, killing one, mailed a letter hours before the shooting that blamed his old bosses for his child pornography conviction, according to police. Existence of the handwritten, two-page letter from Victor Piazza of Warwick was revealed by police Sept. 30, the same day one of the victims, 48-year-old office manager JoAnne O’Brien of Greenwood Lake, died from a gunshot wound to the head suffered Monday. Two co-owners of the Verla International nail polish factory who also were shot survived. Town of New Windsor Police Chief Michael Biasotti said Piazza mailed out a rambling letter to a worker at a Queens coin-operated laundry he ran less than 24 hours before the shootings. Police declined to release the letter, which provides evidence that Piazza was seeking revenge. But the chief described its contents. In the letter, he blames unnamed Verla managers for setting him up for the child pornography found on his work computer in 2004. He was fired after the discovery and pleaded guilty to possession of child pornography, resulting in a sentence of probation. Piazza believed he was headed to jail for violating his probation on the day he walked into the Hudson Valley factory and started shooting, according to the letter. Without ever specifically mentioning the shooting in the letter, Piazza wrote “that is why I decided to do what I’ve done.” “It certainly confirms our theory that this was a planned-out homicide-suicide,” Biasotti said. “He wasn’t just randomly shooting people in the building. He passed many people to get to the people he intended to kill.” Biasotti said Piazza’s belief that he was framed doesn’t hold water. Piazza gave a full confession after his arrest, telling police which particular pornographic movie clips were on his computer and what Web site they were downloaded from, Biasotti said. “I’m sure he’s convinced himself over time that this is somehow the fault of the bosses where he worked rather than his own doing,” the police chief said. The immediate pre-cursor to the shooting appeared to be the discovery Sept. 23 by a probation officer of adult pornographic DVDs and an open pint of vodka at Piazza’s house in Warwick. Piazza was to be arraigned on the probation violation on the day of the shooting. Officials said he could have faced jail time, though it was not a sure thing. As for the other shooting victims, co-owner Mario Maffei, 57, was back home recovering and co-owner Robert Roth, 65, remained hospitalized with a bullet wound to his jaw, according to Verla manager Leo Gesvantner. Biasotti said it appeared that Piazza’s suicide was pre-planned. Maffei, the third victim, managed to throw a chair at Piazza and scramble past him before being shot in the back of the head. Instead of finishing off the wounded man, Piazza saved the last bullet in his .38 revolver for himself, police said. “He went into an office, sat in a chair and shot himself upon our arrival,” Biasotti said.