MONTREAL - Transit officials are blaming human error for a complete shutdown of the metro system that led to the trains starting two hours late Saturday.

The yellow, blue and orange lines returned to normal service at around 7:30 a.m. as did most of the green line, with the exception of the section between Lionel Groulx and Viau, which remained closed until 9:15 a.m.

An estimated 20,000 metro users were affected by the outage, which led officials to offer a fleet of buses to ferry the passengers above ground.

STM Representative Isabelle Tremblay told CTV Montreal that it is believed that an error made during overnight computer maintenance caused the system to go on the fritz.

“There was routine work done on certain computer units overnight, but when the system was re-started, several electric feeders were not operational. It’s our priority to re-establish service first and investigate what happened later,” she said.

“We know that it was human error, an error in the application of the process but we don’t have exact details yet,” she said.

Tremblay said that the cause of this outage was different from other recent breakdowns in the system.

The transit authority conducts overnight maintenance every night between 1:30 a.m. and 5:30 a.m. The operations can involve complex measures that involve up to 500 different professions.