Thunder Bay's fire chief described the blaze at the James Street bridge Tuesday night as "significant," but said there was no indication what caused it or how long the busy bridge would remain closed.
"When [fire crews] showed up from the north side, they found significant flames and smoke on all three lanes of the bridge," John Hay said.
"Our crews saw flames that were six feet high."
Hay said the call came in just after 7 p.m. Tuesday night.
By 9 p.m. the fire as "under control" and crews were in the process of cleaning up and assessing damage to the bridge.
As for the cause, Hay said it was "way too early to be asking" that question.
"That won't be 'til [Wednesday] — at the earliest," he said.
Hay said there were no vehicles on the bridge at the time of the fire, and no injuries were reported.