Several dozen suspected members of the Montreal Mafia could walk free after federal prosecutors decided to halt proceedings against them today.

But prosecutors are vague about the reasons for the decision, raising speculation they are uneasy about evidence gathered using a secretive and controversial surveillance technique.

Between 2014 and 2016, RCMP officers boasted of the success of Operation Clemenza, a multi-pronged operation that led to the arrests of close to 50 people on charges ranging from drug trafficking to kidnapping to firearm possession.

The operation was hailed as a major blow to the Montreal Mafia, which at one time was considered among the most powerful organized crime networks in the country.

On Tuesday, lawyers for the federal government issued 36 stays of proceedings for those rounded up in Operation Clemenza.

That means prosecution is effectively being abandoned against these suspects, though the Crown has the option of resuming prosecution within a year. Eleven cases will still go ahead.

Crown prosecutor Sabrina Delli-Fraine told reporters outside the courtroom in Montreal that the decision was based on "many factors."

Crown prosecutor Sabrina Delli-Fraine speaks to reporters at the Montreal courthouse on Tuesday. (CBC)

Among them, she said, is a recent Supreme Court decision known as the Jordan ruling, which puts limits on the delays an accused can face before going to trial.

She also mentioned that the evidence gathered by the RCMP raised "unprecedented legal questions," but declined to say more.

BlackBerrys key to probe

At the time of the first arrests in 2014, the Mounties spoke of having intercepted around one million messages between BlackBerry users (so-called PIN-to-PIN messages).

"This is the most important interception of its kind in the context of a major investigation in North America," Insp. Michel Arcand had said then.

In court documents unsealed last year, it was revealed the RCMP relied on a "covert intercept unit" as part of Operation Clemenza.

The unit used a device called a "mobile device interceptor" — sometimes known as a Stingray, after a popular brand name — which mimics a cellphone tower. It tricks cellphones within a given radius into sharing information with the device operator, such as location, data transmissions, texts, emails and voice conversations.

An undated photo provided by the U.S. Patent and Trademark Office shows the StingRay II, manufactured by Harris Corporation, of Melbourne, Fla. The mobile device interceptor is used for surveillance purposes. (U.S. Patent and Trademark Office/Associated Press)

The RCMP have, in recent years, gone to great lengths to keep secret details of the device and how it's used. Several media organizations, including the CBC, have challenged these attempts in court.

The use of mobile device interceptors, or MDIs, has alarmed privacy advocates, who point out they are unable to single out specific cellphones, but capture all cellphone data within a given area, even those of people not targeted by an investigation.

Such sweeping capabilities reflect that MDIs were initially designed for intelligence gathering, and not forensic investigations, said Christopher Parsons, a privacy expert at Citizen Lab.

"Police may be concerned they will not hold up [in court]," Parsons said. "There might be questions or debates as to the way in which data is collected, at least using some of the older devices."

Secret no more

Until last year, Crown lawyers had been able to avoid revealing details of how investigators gathered evidence as part of Operation Clemenza.

Seven people pleaded guilty in March 2016 for their role in the killing of Salvatore Montagna, a high-ranking member of a New York crime family gunned down near Montreal in 2011.

The guilty pleas meant the Crown did not have to disclose evidence to the defence. But court challenges by the media later brought out details of the surveillance operation that allowed them to solve Montagna's murder.

As part of those subsequent disclosures, it was revealed the RCMP combined information gathered from the MDIs with a global decryption key that allowed them to read normally encrypted PIN-to-PIN BlackBerry messages.

Salvatore Montagna, a high-ranking member of a New York crime family, was shot dead outside Montreal in 2011.

It is not clear how the RCMP gained access to the key.

The degree to which such considerations influenced the Crown's decision on Tuesday to seek stays of proceedings is unclear.

​NDP Leader Tom Mulcair asked the prime minister during question period why the proceedings were halted. Justin Trudeau was evasive in his answer.

"We are constantly working to improve our justice system, to ensure criminals are pursued and face consequences for what they do," Trudeau said.

"There is always work to do to improve that system."