For about a half an hour on Wednesday morning, all four lines of the metro system stopped running.
The green, orange, yellow and blue lines all shut down at around 10:30 a.m. on Wednesday.
It was the second system-wide interruption in as many days. On Tuesday afternoon, just before rush hour, a similar problem paralysed the metro system.
The STM says the problems on both days had to do with a breakdown of the system's communication network.
On Tuesday, it was the wired phones, including the emergency phones on the platforms.
On Wednesday, it was the wireless network used by inspectors and maintenance crews.
An STM spokesperson says in such instances, a strict security protocol kicks in, which requires a total system shutdown.
The STM is trying to determine whether the two incidents are linked.
Both days, the STM promised to restore service within a half an hour, which it did.