"Down the hall I could see tons of smoke, and I know by the time the engineers and professors got out, the whole hallway was covered in smoke, and they could barely see anything," Hayley Gershon said.

Three students, a BU police officer and a firefighter were treated and transported for smoke inhalation. Their injuries are non-life threatening.

Students weren't sure how to react when they first heard the fire alarms Friday.

"I originally thought it was fake," Jenna Perlman said. "I heard the 'ding ding ding' of the fire alarm -- I thought it was the ventilator making noise."

The Boston Fire Department said the room where the first started is soundproof and heavily insulated. The Boston fire chief estimated damages at $500,000.00. Fire investigators did not immediately say the fire's cause or point of origin.

"There was a lot of water damage. So with it being on the third floor, it came down on the second and first floor. We're going to have to assess to see how much repair we are going to need to do to get students back into the building," BU Deputy Police Chief Scott Pare said.