Three people were injured when a massive fire — triggered by a natural gas explosion — tore through a Far South Side apartment building on Saturday morning in the East Side neighborhood.

Crews were called about 6 a.m. to the blaze in the 9600 block of South Ewing, and it was upgraded to a second-alarm fire about 35 minutes later, according to the Chicago Fire Department.

Witnesses confirmed that the fire was started by an explosion, CFD spokesman Larry Langford said.

The cause of the explosion was ruled accidental due to a buildup of natural gas, Langford said Monday afternoon. A gas valve was inadvertently opened and caused gas to escape, which eventually ignited.

One person was taken to a hospital in critical condition with severe burns, fire officials said. Another person was injured when they jumped from the building, and a third suffered minor cuts.

More than 100 emergency crews were called to the scene. Firefighters rescued two dogs from the building.

By the time the fire was brought under control about an hour later, the back of the building had collapsed.