Two men were recovering from burns and smoke inhalation following a fire at an abandoned building early Monday in Fuller Park on the South Side.

Firefighters responded about 2:30 a.m. to the building at 4721 S. Princeton Ave., where they found the two victims, according to the Chicago police and fire departments.

A 45-year-old man was taken to University of Chicago Medical Center, where was listed in serious condition with burns and smoke inhalation. Another man, 68, also suffered smoke inhalation and was taken to the same hospital, where he was in fair condition, officials said.

The men had been unlawfully occupying the building, according to the fire department. The number of people staying in the building wasn’t available.

The fire was put out about an hour and a half after firefighters arrived, officials said.

Fire officials are investigating how it started.