Lowell firefighters rescued a critically injured woman Monday morning from a large fire that destroyed one downtown business and damaged another.

The fire started in the basement of a thrift shop on Central Street before 4:30 a.m., Lowell fire chief Phillip Charron said. The flames also spread to a cell phone store next door.

Charron said it was a "very difficult fire" for crews to put out.

Lowell firefighters encounter heavy smoke

"They found heavy black smoke coming out of the first floor," he said. "The fire spread from the basement and comes up, some of the connections to the basement don't have doors, so there was nothing to physically stop it."

The building is not residential and no stores were open at the time, according to the fire chief, but rescuers had to pull a woman from the fire and take her to the hospital. It's not clear why the woman was in the building when the fire started.

Hours later, crews were still putting water on the scene. Investigators with the state fire marshal's office were also looking at the damage.

The cause of the fire is still under investigation.