Police say an incendiary device was used to set the $1-million fire at a construction site next to Children's Hospital earlier this week.
A police spokeswoman said Monday no arrests have been made.
The fire, which broke out at about midnight last Wednesday at the site of a $59-million diagnostic-imaging centre, forced the evacuation of almost 100 children, including patients in the neonatal and pediatric intensive care units.
The seven-storey building is scheduled to open next spring and will include a new pediatric MRI, new operating room angiography suite and several other units.
There will also be a heliport on the roof to transfer patients directly to Health Sciences Centre instead of to the airport and then by ground ambulance.
"Any case of arson is disappointing," the Winnipeg Regional Health Authority said in a prepared statement. "We remain grateful that there were no injuries and that our staff, Winnipeg Fire Paramedic Service and Winnipeg Police Service responded so quickly and professionally."