CHICAGO (CBS) — An explosion and roof collapse trapped two workers under six feet of concrete.

CBS 2 is learning new details about what may have sparked the blast.

It’s hard to believe that no one was killed inside a water reclamation plant on the far South Side.

2 Investigator Dorothy Tucker has been on the scene for hours and reports on what she’s learned about the cause of the explosion.

The preliminary cause is methane gas. The question is what ignited it? A blowtorch? A cigarette? What sparked the explosion?

A massive explosion Thursday morning at the Metropolitan Water Reclamation Plant sent the concrete roof plunging to the ground, landing like a pancake.

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

Getting the second worker was a challenge.

“Buried and entombed. The companies had to dig down six feet and then tunnel away another 20 feet to the victim,” said Chicago Fire Department Commissioner Jose Santiago.

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 Hospital.

Methane gas is naturally released from the sewage that’s treated at the plant. It had somehow built up. But what flame sparked the explosion?

“I cannot tell you if they were using a torch or anything else,” said Alicia Tate-Nadeau of the Office of Emergency Management.

According to OSHA, there are no records of past complaints against the plant.

A statement from the MWRD:

“The MWRD applauds the City of Chicago’s emergency responders for their commitment and expertise in rescuing and treating our workers.  We will thoroughly investigate this incident to determine how this happened and identify measures to prevent something like this from happening again. Our trades and plant staff can work in potentially dangerous situations in order to protect the safety and health of the people of Cook County. Those injured today and their families are in our thoughts and we hope that they make a swift recovery from their injuries.”

Dorothy Tucker