Police have arrested 13 people for allegedly topping up the public transit Opus cards using fake or cloned credit cards.
The arrests for fraud happened over the past three weeks.
Police say the thieves would buy the blank Opus cards for $3.50 each, and then go to one of the automatic machines to charge the card with a fraudulent credit card.
A few weeks ago, banks started noticing more cards being topped up with fake or cloned credit cards.
"It's really very simple for someone who has the knowledge and the equipment to reproduce magnetic cards," said Rene Allard with the Montreal Police.
Police say they believe the cards are mostly sold over the Internet. Police say in many cases, the person says they have to sell the card because they are moving.
The internet postings have helped police confiscate more than 200 cards and thousands of dollars.
"In one event, we found $45,000 in cash in the car of the suspect," said Allard.
Once a card is found, the STM can cancel it. Police can use the date and time stamp on the card from when it was charged up to track the thieves.
Police anticipate more arrests. They say anyone caught using one of the falsely-loaded cards will be prosecuted.
