The STM is investigating several people who hid in metro tunnels, opened doors between metro cars, and sat in a train driver's booth.
They also set off fireworks and used sparklers inside metro tunnels.
The events were all captured on a video the group posted online on Sunday.
The video includes footage of the group lifting a manhole cover and descending a ladder that leads to STM service tunnels.
On that video a woman spots the intruders while they are sitting in a train driver's booth, and can be heard yelling at them to get out of the train.
Transit officials said the events captured on camera are "dangerous and illegal" and they are seeking to identify the people in the video.
