The million-dollar blaze broke out last week at a construction site at the Health Sciences Centre next door.
No one was hurt.
A police spokesman says the discovery changes the nature of the investigation.
Police aren't using the word "arson," but are instead describing the fire as "incendiary."
However, they say there is no evidence to suggest an explosive device was used.
"We're not specifying any of those things right now, but we're acknowledging, based on the information we have so far, it does appear that the fire was deliberately set," said Const. Jason Michalyshen
The fire seriously damaged the diagnostic building under construction.
(CJOB, The Canadian Press)