Authorities have determined the cause of an explosion at a sewage treatment plant that left 10 people injured on Thursday morning.

According to the Office of Fire Investigation, the explosion was caused by the use of a welding torch in an area of the building with significant methane gas present. The torch caused the gas to ignite, causing an explosion and shock wave that lifted the roof of the structure, the office says.

Later Friday, the Chicago Fire Department announced it had ended its investigation of the incident at the plant, and insisted that it "not be assumed the operator of the torch bears responsibility" for the blaze.

Several workers were trapped by debris from the blast, and rescue workers were forced to dig through the roof of the structure to get to the victims inside.

NBC 5 Investigates has been looking into the timeline of the collapse and rescues. We’ve also learned there have been no OSHA violations at the plant. NBC 5’s Phil Rogers reports.

According to the Water Reclamation District, eight of the 10 people injured in the explosion have been released from local hospitals.

"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," the agency said in a statement.

The plant is one of seven wastewater treatment facilities owned by the Metropolitan Water Reclamation District of Greater Chicago.