A Montreal janitor who stumbled on boxes of fake credit cards and ID while hunting down a leaky pipe was the key to a major fraud bust that is one of Canada's largest national security investigations.

Mark Duke, a Montreal janitor, found piles of counterfeit ID material while trying to fix a leaky pipe. (CBC)This week police revealed more details about their crackdown on international fraud, which resulted in 30 arrests after investigators raided several locations in Montreal and Ontario, including a handful of daycares.

Operation Pre-Empt is the largest counterfeit document bust in Canadian history, according to police who worked on the probe.

And it started with Mark Duke, a janitor who works in an apartment building in downtown Montreal.

Duke said he was called to fix a leaky pipe in December 2006, and while tracking down the source, used his master key to enter a unit in the building.

Inside, he found boxes filled with blank credit cards, passports, printers and other equipment that could be used to produce identification.

The apartment was sparsely furnished with just "an inflated bed, a pillow and a blanket" Duke told CBC News.

He contacted police with his findings and that prompted the three-year investigation.

The 30 people arrested are in custody, awaiting trial in the new year.