MONTREAL - Police say they've figured out the motive for a perplexing string of firebombings against mostly Italian cafes in Montreal: street gangs demanding a share of drug profits.

Months after the arson attacks quietly ended and following a series of arrests, police offered a theory Wednesday for the 18 firebombings that baffled the city.

A befuddled police force was floating a wide range of theories just a few months ago. They included a possible feud pitting Montreal versus Toronto mobsters, a local battle between Mafia families, competing clans in the same family, or a battle between the Mafia and street gangs.

Police are now confident the attacks came from street gangs seeking a greater slice of the narcotics trade.

"They wanted a cut on every (dollar) of drug sales," said Deputy Police Chief Jacques Robinette.

Robinette said arrests have been made in five of the attacks, but 13 cases remain unsolved.

He stressed that the attacks were against the specific cafes and not against the broader Mafia, as gang members tried to apply pressure on businesses where they presumed drugs might be sold.

None of the incidents resulted in damage or injuries.

In the initial aftermath of the attacks, police admitted they were stumped by the case. The shop owners weren't talking much.

During a semi-annual briefing on the state of gang activity in the city, police addressed a wide range of other issues.

A recent attempt on the life of presumed street-gang leader Ducarme Joseph that left two people dead in an Old Montreal clothing boutique was -- despite earlier speculation of Mafia involvement -- strictly a gang affair, they said.

Robinette said police are working to understand what is transpiring with the Montreal Mafia, where a number of recent killings and kidnappings have signalled a shift in the underworld power structure.

Quebec provincial police, the RCMP, police in Ontario and in the United States have all been enlisted, Robinette said. All hypotheses are being considered, nothing is being discounted, Robinette said.

"I've heard and read it's a war, but a war against who?," Robinette said.

"The Sicilians and the Calabrians? For now, it's the Sicilians that are getting taken down, no Calabrians. You need (opponents) for a war, right now there is no war."

But Robinette said street gangs appear to be a player in the shift, with an unidentified suspect in the slaying of Nick Rizzuto Jr. believed to be gang-affiliated.

Bikers have used street gangs in the past to do their dirty work -- but not the Mob, which generally handles its own affairs, Robinette said. He called the incident a rarity.

Police also noted a major spike in the number of illegal weapons seized so far this year.

The number of weapons seized in 2010 stands at 144, compared to 168 in all of 2009.

"Everybody wants to have their own weapon to protect themselves, when we interrogate them. That's what they say," Robinette said.

He added, however, that police have also seen a huge decline in the number of street-gang-related violent crimes -- 14 homicides in 2007 compared to just two this year.