The rear of the building in the 9600-block of South Ewing Avenue collapsed during the initial stage of the fire around 7 a.m. after a possible explosion, the fire department said.
One man was critically injured with burns on 27 percent of his body. He was transported to University of Chicago Medical Center. One man had injuries from jumping out of the building and another man had minor lacerations, the fire department said. A girl was treated for minor injuries at the scene.
The three-story building has six units. Neighbors said they heard an explosion and that they felt their homes shake.
"The whole house shook. I actually thought someone was kicking in my door, that's how loud it was. I have two dogs and they started going crazy," said neighbor Brandon Franklin.
"We heard an explosion, we thought someone hit the house it shook the house and about five minutes later I heard the sirens, I came out here and the next thing I know the whole thing was up," said neighbor John Castillo.
The fire chief on the scene said the building was already compromised, making it even more dangerous for the more than 100 personnel fighting the blaze. People's Gas personnel also responded.
"The gas meters were burned off, so we had a free-flowing gas, We had to shut the gas off to the building before we could actually really extinguish a lot of these fires," said Chicago Fire Department District Chief of Training Donald Hroma.
Two dogs were also rescued by firefighters and about 12 people have been displaced.
In a statement, Peoples Gas said: "This morning, People Gas was called to an explosion at 9650 S. Ewing to turn off the gas. We secured the gas by closing a supply valve outside the building and at the b-box in the parkway. We worked with the Fire Department and our crews have left the scene. The City will not allow anyone in the building for safety reasons until Monday. Earlier, Peoples Gas technicians responded to a gas leak call at 12:45 a.m. on the second floor at 9650 S. Ewing. They found a leak at a faulty appliance valve of a stove. They disconnected the stove, notified the tenant and informed them that they needed to have it repaired."