MONTREAL - It began about a year ago with a brazen midday strike against a Mafia don's eldest son on a grungy Montreal street _ an attack that sent a message nobody was untouchable.

That murder set off a one-sided, year-long massacre of the once-formidable Rizzuto clan.

With their charismatic leader Vito Rizzuto behind bars in the United States and with a leadership vacuum, the clan's 30-year rule over Montreal's underworld appears over.

The systemic killings of its hierarchy in 2010 _ punctuated by the surprising execution of the 86-year-old patriarch Nicolo Rizzuto Sr. in his own home _ have left the group decimated.

Who is behind the bloodletting isn't clear. But police and pundits agree on one thing: the family's hold on power is tenuous, at best.

"It's 4-0 and there isn't a lot of the game left to play,'' says one veteran police source.

"Either they have to mount some sort of comeback or it's a done deal.''

The primary motive behind the bloodletting isn't too hard to figure: money and power. Montreal remains the crown jewel for the Italian Mafia in Canada.

But revenge is another element police haven't entirely ruled out. The Calabrians were erased in the city in the late 1970s when the Rizzutos rose to power. The Violi brothers _ three of them _ were killed in brutal fashion.

Vendetta is a possibility and hard to ignore given the symmetry between the current spate of killings and kidnappings and those from 30 years earlier.

Police brass had anticipated a bloody 2010 when Nicolo Rizzuto Jr., eldest son of jailed reputed Mob boss Vito Rizzuto, was killed in late December 2009.

Next came the kidnapping last May of Paolo Renda, a brother-in-law of Vito and the "consigliere'' or No. 3 in the organization.

In late June, Agostino Cuntrera, another man tabbed to fill the leadership vacuum, was assassinated in front of his food-distribution business along with a bodyguard.

Finally, Nicolo Rizzuto Sr. was felled by a single shot from a marksman lurking in the woods behind his luxury home. He was killed in front of his family in what authorities consider the surest sign yet that the Rizzuto clan is done.

Officially, Denis Mainville, a commander heading the Montreal police organized crime and anti-gang squad, will say only that the situation is destabilized and change is taking place.

Mainville said people linked to the Mafia were warned their lives were in danger, but that they shooed away police or armed themselves to the teeth in response.

Montreal police Cmdr. Clement Rose, who heads the major-crimes squad investigating the slayings, says he's hopeful someone will turn informant.

"We don't have a crystal ball,'' Rose, a veteran cop who worked on the anti-biker unit until the late 1990s, said in an interview.

"When you go to a scene and there's not much there, no video, no one wanting to talk, we can't do more than what's being asked of us.''

One thing that is clear is that all the ruckus is not good for business.

"The Mafia is more dangerous when they don't shoot, because that means the Mafia has a Rolodex that is up-to-date, a network of contacts, a network of relations,'' says Antonio Nicaso, who has written extensively about the Mafia.

That's when violence takes a back seat to money laundering, drug trafficking and investments in legitimate business.

"When there is violence, it's a sign that there is a problem and the violence attracts media and police.''

Nicaso said the answer to "who?'' lies in a combination of "old enemies and new rivals'' and a blessing from crime families based in New York.

And the rise of the Calabrian Mob _ based in Ontario _ isn't ruled out.

"If it's true that the Ndrangheta (Calabrian factions) that are based in Toronto are moving into Quebec, that has a lot of significant implications,'' said criminologist Stephen Schneider.

"One group has significant power in Ontario and Quebec and a significant power base back in Europe because they are the biggest cocaine traffickers in Europe right now, so they could wholesale on a huge scale.''

Seemingly up for grabs is an extensive bookmaking operation in addition to lucrative drug-trafficking options worth billions.

A Radio-Canada investigative report recently revealed that the Rizzuto clan was receiving a five per cent cut for arranging construction contracts.

"You have hundreds of criminal groups in Quebec and anyone of those could be consolidating with someone else, making this move,'' Schneider said.

Police say a rash of Molotov cocktail attacks on mostly Italian-style cafes in Montreal appears to be a street war for drug turf once controlled by the Rizzuto clan.

"We can see the people who want to challenge the Rizzutos on different levels,'' Nicaso said. "There are people (who) want to replace the Rizzutos on the street.''

It seems likely that operations are being divided up piecemeal.

"What you're going to see is a highly fractionalized criminal underworld in Quebec where there is going to be a lot of smaller cocaine and hash sellers,'' said Schneider, adding those alliances might be short-lived.

"The trend right now is that you don't see those relationships last for very long,'' he said.

The shift in power comes as ties between the Mob, big business and political elements are under intense scrutiny. Demands for Premier Jean Charest to call a public inquiry into allegations of corruption in the construction industry are virtually unanimous.

Some experts believe this might be the right time to call such an inquiry.

"If they're done right and (the inquiry) dovetails well with police investigations, then they can be really effective,'' Schneider said.