CHICAGO (CBS) — Crews using a blowtorch in a building that had filled with methane gas set off an explosion at a Far South Side water treatment plant on Thursday, bringing the roof down, trapping two workers and injuring a total of 10, according to Fire Department officials.

A massive explosion Thursday morning at the Calumet Water Reclamation Plant at 400 E. 130th St. sent the concrete roof plunging to the ground, landing like a pancake.

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A Fire Department spokesperson said the Office of Fire Investigations determined workers were using a blowtorch “in an area with significant amount of methane gas present,” causing an explosion in the plant’s sludge concentration building. Methane is an odorless gas naturally released from the sewage that’s treated at the plant.

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Sources familiar with the investigation told CBS 2 maintenance workers were removing the top of a large bolted metal sludge tank, to begin the routine process of moving sludge from one tank to another. One of the nuts on the tank would not budge, so a worker used a cutting torch to remove the bolt. It is believed methane gas that had been contained by the tank was ignited by the cutting torch, causing a massive explosion.

Workers told investigators they saw a flame, and then a shock wave that lifted the building’s roof, and caused it to collapse.

“We consider it a miracle no-one was killed. They were just lucky. The roof rose up several feet due to the blast then came down,” Fire Department spokesman Larry Langford said.

Langford also stressed that the worker who was using the torch should not be blamed for the explosion, because he was working as directed. Questions still remain as to what caused the buildup of methane and why a torch would have been used in those conditions.

Ten workers were inside the building at the time of the blast. Eight managed to climb out and were taken to hospitals. But two were trapped, forcing rescuers to search the rubble.

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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 Fire Commissioner Jose 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 slabs of concrete.

Once they reached him, 30 specially trained rescue responders were called to the scene to free him. Working non-stop for nearly two hours, the man was finally removed from the rubble and eventually transported by helicopter to the University of Chicago trauma center to be treated for a broken jaw and a badly fractured leg.

To avoid having to amputate his leg, the firefighters who worked to free him contacted doctors at the University of Chicago, and paramedics provided him with intravenous fluids and medication to treat a possible compartment injury from the intense pressure of the concrete.

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

Metropolitan Water Reclamation District officials said eight of the ten injured workers have since been released from the hospital.

“We wish them all a speedy recovery,” the agency said in a statement. “The MWRD is extremely grateful for the tremendous effort put forth by the Chicago Fire Department and other emergency responders in extricating the two trapped workers and attending to all of the injured.”

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The agency said the Calumet plant is still operating after the explosion, and water treatment is continuing as normal.