New York- An upstate sex offender – fired after he was busted for having child porn on his office computer – yesterday shot three former bosses and then killed himself in the perfume factory where he once worked, police said.

All three victims and the gunman – the company’s former quality-control officer – were shot in the head inside the offices of the Verla International perfume and nail-polish plant in New Windsor, Orange County, authorities said.

Police said Victor Piazza, 55, walked into the building at about 12:25 p.m. and started blazing away without warning at the executives he blamed for firing him.

Terrified workers heard the shots and started fleeing the building on Temple Hill Road.

“Everyone just started screaming, ‘Run, run!’ ” said factory worker Alfredo Amigon.

“Everyone was running with their hands over their heads. People were screaming, ‘They’re shooting! They’ve shot Mario.’ ” Mario Maffei was the company’s co-owner.

When cops arrived at 12:30 p.m., they found Maffei, 57, lying in a pool of blood in the company parking lot.

Inside, they discovered his partner, Robert Roth, 65, and office manager JoAnne O’Brien, 48.

Piazza’s body was found in a chair in a second-floor office.

Police said his fatal head wound was self-inflicted.

“It was horrible,” New Windsor Police Chief Michael Biasotti said of the bloody scene.

He said Piazza, who was fired in February 2004, believed his victims were responsible for his termination.

Piazza was sacked when New Windsor police busted him after company officials notified cops that child porn had been found on Piazza’s company computer, Biasotti said.

Piazza, who lived with his father in Warwick, pleaded guilty to the charge and was given 10 years’ probation.

A man identifying himself as Piazza’s father told The Associated Press, he saw no signs his son was angry with his former employer. “Oh, no, nothing like that,” he said.

Two of the victims were rushed to St. Luke’s Hospital in Newburgh.

One of O’Brien’s relatives said, “They’re all OK and they’re all going to be OK.”

But a hospital spokeswoman said O’Brien was in “very critical condition,” and Maffei was in “critical condition.”

Roth was airlifted to Albany Medical Center and was in stable condition, police said.

Leo Gesbantner, another factory employee, said O’Brien “beat breast cancer a few years ago and she’s going to beat this now.” He described the co-owners as caring bosses. “If I ever had a problem I could always go to them directly,” he said.

Verla International closed for the day after the shooting, but may re-open today, Gesbantner said