The man, identified as Pierre Lebrun, opened fire on transit workers with a high-powered rifle Tuesday in what police described as a premeditated attack.
In addition to the dead, two workers were injured. Lebrun fired nine rounds of ammunition and police say he was carrying 36 more.
Inspector Ian Davidson said officers found a note at Lebrun's home, outlining his plans for the assault and identifying employees he liked and those he bore grudges against.
Police declined to say if the dead and wounded employees were on the list.
The president of the transit workers union, Paul Macdonnell, described Lebrun as a "very sick" man with chronic mental problems. He had been referred for psychiatric treatment two years ago.
"Clearly he was a troubled individual who had some difficulties in the work environment," Davidson said, but police would not confirm any details about Lebrun's mental condition.
Lebrun, 40, was fired in 1997 after a fight with another worker, then reinstated as result of a grievance filed by his union. He quit earlier this year.
Police said Lebrun the purchased rifle used in the shooting back in 1992. They said he had no criminal record.
Authorities described how the deadly scenario at the garage played out. Police say Lebrun drove up to the bus depot in mid-afternoon, killed one worker immediately, shot his other victims in the machine shop, then climbed to an upper floor and killed himself.
"We heard the shots," said garage attendant Marcel Gleason. "The foreman came through telling us to run outside; somebody was trying to shoot everybody, so everybody ran out. It happened so fast."
Pierre Servant, a bus driver, said he was walking through the door when someone yelled that a gunman was inside.
"The man was yelling to call 911. But before he could get out he was shot in the back or the side and fell down. At that point everyone scattered," he said.
Mass murders are unusual in Canada, although there have been worse multiple slayings. Marc Lepine killed 14 women at the University of Montreal in 1989.