It says refurbishment work being done on old pipes near McGill University might have shaken the soil, destabilized the mechanisms holding the pipes in place, and knocked them loose in a spot where they were connected together.
Nearly 40,000 cubic metres of water gushed from the site and down the slope of Mount Royal, making that area temporarily take on the appearance of a raging rapid.
Water spilled out for two hours on Tuesday night, knocking some people to the ground but causing only minor injuries.
The flood does appear to have caused damage to buildings. Classes have been relocated from some buildings at McGill that have been closed indefinitely.
It's not yet clear who will be held financially responsible for the damage. A statement from the city today says it will not conduct any interviews on the matter.
The city says its theory about the flood cause comes from the municipal water service, which completed an analysis yesterday.