CHICAGO (CBS) — CBS 2 has learned it was a blow torch that sparked an explosion Thursday at a water plant.

The blast was so powerful, it lifted the roof completely off a water treatment plant.

CBS 2’s Chelsea Irving reports from the University of Chicago Hospital where two of the workers are recovering.

One of the two workers at the hospital is the iron worker who was using the blowtorch that investigators said sparked the explosion.

But officials added he did nothing wrong, doing his job like any other day.

Exactly what went wrong is still under investigation.

Fire officials said the worker was using a blowtorch to open a sludge tank at the water reclamation plant Thursday when something sparked an enormous tank of methane gas.

“There was a torch present down in the facility. Its purpose was to remove a frozen bolt which is common practice,” said John Murray of the Metropolitan Water Reclamation District.

OSHA is investigating the exact cause which officials speculate may be mechanical. The Metropolitan Water Reclamation District is also hiring its own forensic investigator.

The worker lucky to be alive.

“There are no words for that. As soon as I saw the news alert, I had a really bad feeling,” said Carolyn Grisko who saw the alert about her brother Thursday. Now, she and his wife are at his beside as he struggles with injuries including a shattered leg, broken spinal disks, breast bone, shoulder, wrist and nose.

“Our family is so grateful to the firefighters and paramedics who literally put their own lives at risk for my brother and his friend,” said Grisko.