Nicolo Rizzuto Sr., in the fedora, is surrounded by family as he leaves the January 2010 funeral of his slain grandson Nick Rizzuto. The elder Rizzuto was gunned down in his home in northwest Montreal on Wednesday. (Paul Chiasson/Canadian Press)

The patriarch of Montreal's most notorious Mafia family — Nicolo (Nick) Rizzuto — has been killed in his home in a slaying that caps months of escalating attacks against the embattled crime clan.

Rizzuto was inside his luxurious family mansion in northwest Montreal when he was shot around 5:40 p.m. ET Wednesday.

Police weren't revealing many details but said one or several suspects were involved, and that the shots could have come from inside or outside the home, located on Antoine Berthelet Avenue in the city's Ahuntsic-Cartierville borough, near the Rivière-des-Prairies.

Rizzuto, considered the most powerful figurehead of Montreal's infamous Mafia, was 86.

Two women in the house witnessed his shooting and someone called 911, police commander Denis Mainville said.

Rizzuto was taken to nearby Sacré-Coeur Hospital, where doctors declared him dead. The women were treated for shock and are being held in an undisclosed location for security reasons, Mainville said.

Police blocked the street and investigators with the major crimes squad spent the night combing the area, where several members of the Rizzuto clan have homes.

Rizzuto family under attack

Authorities beefed up security across the city in preparation for any possible retaliation, but pinpointing the source of the attack on Rizzuto will take time because "every theory is possible," Mainville said.

Nicolo Rizzuto, seen in an archive photo, was on probation when he was killed. (Canadian Press)Police have been preoccupied for months with the apparent power struggle in the Mafia underworld, Mainville admitted, a tug-of-war marked by calculated hits on the Rizzuto clan.

Rizzuto's grandson Nick Jr. was shot dead on a Montreal street in broad daylight last December. Rizzuto lieutenant Paolo Renda was kidnapped in May and is still missing. This summer, Agostino Cuntrera, another Rizzuto associate known as the "Seigneur" or lord of the St-Leonard borough, was gunned down with his bodyguard in front of his warehouse.

Wednesday's shooting heralds the demise of a long-dominant criminal organization that, for decades, lorded over the underworld with few rivals.

Nicolo Rizzuto was among dozens of suspected Mafiosi rounded up in 2006 after an exhaustive police investigation, dubbed Operation Colisée, which involved more than one million hours of wiretap. He and others arrested were charged with a litany of offences, from drug smuggling, money laundering to extortion and bookmaking.

But Rizzuto emerged from the court procedures with a two-year suspended sentence and probation after he pleaded guilty to gangsterism-related charges.

Vito Rizzuto, shown in an archive photo, is serving a 10-year sentence in a U.S. prison. (Canadian Press)

Authorities were forced to admit that despite hundreds of thousands of hours of wiretap, they could not directly implicate Rizzuto to the majority of charges.

Rizzuto later pleaded guilty to tax evasion, receiving probation and a stiff fine.

His son Vito is serving a 10-year sentence in a U.S. prison for his role in a 1981 triple murder at a Brooklyn social club. Vito Rizzuto was extradited to the U.S. in 2006, and pleaded guilty to racketeering charges.

At his sentencing hearing, he admitted he was at the scene of the murder and was armed.

Mafia watchers believe Vito Rizzuto's extradition, and the 2006 dragnet against Montreal bosses, created a power vacuum that enticed rival families or crime rings to bid for their turf.

The lines are blurry in the criminal underworld, said Mainville, with many groups — from street gangs to Mafia families — maintaining complex "business relationships," even as rivals.