CHICAGO (CBS) — Two people were trapped, and eight others were injured, after an apparent methane explosion caused the roof of a building to collapse at a water treatment facility on the Far South Side.

Fire Commissioner Jose Santiago said firefighters were notified of the explosion at about 10:50 a.m. at the Calumet Water Reclamation Plant at 400 E. 130th St. Ten people were inside the building at the time, and two were trapped in the rubble.

The Metropolitan Water Reclamation District of Greater Chicago said the explosion happened in the plant’s sludge concentration building. A Chicago Fire Department source said workers using a blowtorch hit a gas line, but MWRD officials said they could not confirm that.

It took firefighters about 20 minutes to free one of the workers who was trapped, but the second was “buried and entombed” under thousands of pounds of concrete, according to Santiago.

Fire Department Chief of Special Operations Tim Walsh said firefighters dug down about 6 feet into the rubble, and then tunneled about 40 feet to reach the second trapped worker, whose legs were trapped by massive concrete beams. To avoid having to amputate his leg, firefighters contacted doctors at the University of Chicago trauma center, and paramedics provided him with intravenous fluids and medication to treat a possible compartment injury from the intense pressure of the concrete.

It took firefighters about two hours to get the second trapped worker free.

“They did a phenomenal job,” Deputy FIre Commissioner William Vogt said.

Walsh said the worker who was buried suffered a broken jaw and a badly fractured leg. That worker was airlifted to the University of Chicago after he was rescued.

MWRD President Mariyana Spyropoulos confirmed there was a buildup of methane in the building before the explosion, but said the cause of the blast was still under investigation.

She said she could not confirm or deny reports that workers were using a blowtorch at the time of the explosion. She said all 10 workers who were in the building at the time were inside, not on the roof. While the sludge concentration building was offline at the time of the explosion, Spyropoulos said the rest of the plant remained operational after the blast.

Fire Department officials said the cause of the explosion was being investigated by the Office of Fire Investigations, the Chicago Police Bomb & Arson Unit, and the MWRD.

Illinois OSHA officials said they do not have any records for past complaints against the Calumet Water Reclamation Plant.