LOWELL (CBS) — Two firefighters were hurt while knocking down a large building fire in Lowell. Flames broke out in the three-story building on High Street around 9 a.m. Sunday.

The firefighters, who are part of the Chelmsford Fire Department, were providing mutual aid at the scene. They suffered minor injuries to their ankle and hand.

Fire Chief Jeff Winward said the layout of the structure made this a stubborn fire.

“The building is all cut up, it’s an old building. There’s walls dividing the third floor so we had a tough time getting to the fire for a long time,” he said.

Firefighters were called to High Street around 9 a.m. Sunday. The fire reportedly was first seen on the porch but spread up the building to the roof (WBZ-TV)

The building includes both seven residential and three commercial units.

Winward said the building’s smoke alarm system was working so everyone was able to get out before firefighters arrived.

“The first report we got was for a fire on the back porch and it looks like the fire went up the back of the building and went across the top of the building. The whole roof was pretty much involved in fire at one point. So the fire went to four alarms,” explained Winward.

The cause of the fire is under investigation.