Firefighters were called to the site of the future Peel Memorial Centre for Integrated Health and Wellness just before 7:30 p.m. on Monday.
The fire burned for hours.
Fire investigator Gary Jarrett said that it became clear on Tuesday that "there is a lot more damage than what we thought" between the first and second levels of the structure.
The investigation into the fire damage was halted by the Ministry of Labour on Tuesday, over concerns that the heat may have weakened some of the concrete in the structure.
There is also concern that some of the structure may need to be torn down before work can start again.
"This is devastating to the city of Brampton," Jarrett said.
Building the new hospital is a $530-million project.
The hospital is scheduled to open in 2016.
Click on the video to see a full report from the CBC's Mike Crawley.