A firefighter who was working a detail in Boston's Financial District is being credited with evacuating people inside an office building as its roof erupted in flame Wednesday afternoon.

Witnesses told WBZ-TV they heard the fire alarm going off inside their offices at 22 Batterymarch but thought it was a drill until the firefighter went floor to floor, knocking on doors.

"We looked outside and what we saw were people taking videos of our building, which is a very scary thing to see," said Alex Gilbertson, who was working on the second floor. "Someone came by and like, knocked on our door and said, 'You have to go downstairs. There's a fire on the roof.'"

The firefighter called the blaze into the department, which arrived and quickly set to work tackling fire on the buildings' wooden and rubber roof. 

"Companies made their way up through the stairwells and hooked up to the stand pipes in there and got lines and water on the fire very quickly. They did a great job putting this thing out," said Boston Deputy Fire Chief James Lonergan. "If it was a windy day, who knows, this thing could have extended down the street."

No one was injured during the firefight. The cause of the fire is still under investigation.

Office workers said they are grateful for the lone firefighter's quick action. 

"Thank God for him. That's big time. I'm just glad it seems like everyone got out and there were other people evacuating," said Ken Bukoski.