Vito Rizzuto, one of Canada's top Mafia leaders who was involved in the infamous 1981 slaying of three American mobsters in Brooklyn, died of natural causes Monday in a Montreal hospital, officials said yesterday.

The 67-year-old mobster, who had been treated for lung cancer, died of pneumonia after he suddenly became ill Sunday night at his home, said his longtime attorney Jean Salois.

A spokeswoman for Sacre-Coeur Hospital in Montreal confirmed Rizzuto's death.

Rizzuto pleaded guilty in May 2007 in Brooklyn federal court to being part of the conspiracy which led to the slaying of three Bonanno crime family captains -- Dominick "Big Trin" Trinchera, Alphonse "Sonny Red" Indelicato and Philip Giaccone -- as part of the power play orchestrated by former crime boss Joseph Massino at a Brooklyn social club.

Rizzuto testified in court his role in the murders was to run into the room armed and say "It's a holdup" and have everyone stand still. Other gangsters opened fire on the three men, said Rizzuto.

After Rizzuto's 10-year federal sentence ended in October 2012, he returned to Canada, Adrian Humphreys, a journalist with the National Post, recalled.

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"He comes out of prison and I think most people had written him off," said Humphreys, who wrote Rizzuto's biography, "The Sixth Family: The Collapse of the New York Mafia and The Rise of Vito Rizzuto," with writer Lee Lamothe.

Rivals had killed Rizzuto's father, one of his sons and other confidantes. But once back in Montreal, Rizzuto started to settle old scores and asserted himself again, both Humphreys and Lamothe said.

According to Humphreys, Rizzuto apparently arranged slayings of rivals in Toronto, Montreal, Mexico and Sicily.

"Before his death he was undisputed the biggest named crime boss -- certainly the biggest name in Canadian crime," said Humphreys.

Defense attorney David Schoen, who represented Rizzuto in the United States, didn't believe the claims that his former client was involved in the Canada slayings.

"He was a very honorable man," said Schoen. "I just like to remember him as I knew him . . . he was always good to his word."