Nick Rizzuto Jr. was gunned down near this Montreal apartment building on Monday. (Graham Hughes/Canadian Press)Mafia experts are warning of a potential organized crime war following reports that Nick Rizzuto Jr., the son of Canada's most powerful mobster, has been killed in what they call a "shocking" murder.

Rizzuto — the eldest son of Vito Rizzuto, the so-called head of Canada's Mafia — was gunned down on Monday in broad daylight on a residential street in Montreal's Notre-Dame-de-Grâce neighbourhood.

The brazen assassination took aim squarely at the entire Rizzuto clan and its leadership, not just Nick Jr., according to Antonio Nicaso, an author and expert on the Mafia.

Attacking an eldest son sends a clear message, and "this is an unprecedented challenge to the power of the Rizzuto clan family," Nicaso told CBC News.

'We should expect a retaliation. A powerful organization like the Rizzutos cannot do nothing after something like this.'—Antonio Nicaso, Mafia expert and author

The Rizzutos are facing serious leadership problems and this killing is indicative of their challenges, Nicaso said. The family has been hard-pressed to keep a firm rein on operations since Vito Rizzuto's arrest in 2004 in connection to three cold-case Brooklyn murders.

With Vito Rizzuto, 63, serving a 10-year sentence in a U.S. jail, the family lost its main mediator who had birthed a "strategic alliance between the Mafia and other criminal organizations … like the Irish Mafia, the Hells Angels, street gangs and Colombia cartels," Nicaso said.

"Many people think the Rizzuto crime family didn't like that the alliance involved street gangs, and they lost contracts, they lost business."

When massive police raids in 2006 targeted many alleged members of the Rizzuto network, a larger void was created in Montreal's underworld, Nicaso contends.

The Colisée police investigation "practically removed hundreds of people from the street and left a vacuum of power in Montreal, particularly on the street level," he said.

Nicaso describes the Rizzutos as a "family of disorganized crime" hitting a low point in their history. "According to police … the Rizzuto crime family is headless, without the boss, divided on certain points, but on most points, an organization that is facing the challenge of powerful street gangs."

It is difficult to predict what will happen in upcoming weeks, but "We should expect a retaliation," Nicaso said. "A powerful organization like the Rizzutos cannot do nothing after something like this."

Victim kept low profile

Nick Rizzuto Jr. was a low-profile member of the powerful family, despite his notorious father, Nicaso said. "We knew he was the eldest son of Vito Rizzuto, alleged Mafia boss of Montreal, but nothing more.

Nick Rizzuto Jr. shown here at the Montreal courthouse in a 2007 file photo. (Benoit Pelosse/Le Journal de Montreal/Canadian Press)"He was involved in the real estate, construction businesses, many things, but he avoided several investigations, including Colisée, the [sting] that removed from Montreal an entire level of management of the Rizzuto clan, practically."

Media reports in Montreal say Nick Rizzuto was visiting a woman friend in NDG when he was shot point blank outside an apartment building just after noon on Monday.

Residents in the neighbourhood said the half-dozen shots sounded like firecrackers.

Paramedics tried to revive the victim before transferring him to hospital, where he was declared dead.

Montreal police would not confirm that the 42-year old victim was Rizzuto, and a family lawyer reached at the hospital also declined to make a statement.

Police said investigators found a weapon near the crime scene but are still hunting for one or more suspects who fled on foot.

Investigators are considering all possiblities, including that the killing may be related to a string of recent firebombings of Montreal cafés.

The shooting is Montreal's 31st homicide this year.

Vito Rizzuto is currently in a medium-security prison in Colorado, serving a 10-year sentence for racketeering in connection to the Brooklyn murders.

The U.S. Federal Bureau of Prisons said Vito Rizzuto could ask to attend his son's funeral.

Spokeswoman Felicia Ponce said the warden at the Colorado prison would have to authorize the request, and Rizzuto would have to foot the travel and security costs.

Ponce said such requests are confidential.

With files from The Canadian Press