CHICAGO (CBS) — Fire investigators have determined a buildup of natural gas at an apartment building in Washington Park likely caused an explosion that leveled the building over the weekend, injuring two women who were inside.

A three-story building at 5845 S. Calumet Av. collapsed around 7 p.m. Sunday night. Firefighters rescued two women — ages 51 and 78 — as well as a dog, after sifting through the rubble.

Tusday afternoon, Fire Department spokesman Larry Langford said investigators have determined the likely cause of was a natural gas explosion.

“The Chicago Fire Department Office of Fire Investigation (OFI) has determined the most probable cause of the building collapse at 5845 Calumet was a buildup of natural gas from an appliance or customer owned piping in the lower level of the building. The resulting ignition caused the structure to fail,” Langford said in an email.

Officials at Peoples Gas assisted the Fire Department in the investigation.

The building was only a few years old when it collapsed. Neighbors who heard and saw the building come down couldn’t believe their eyes.

Angela Brooks said it sounded like a transformer blew around 7 p.m. When she looked out the window, the building was leveled. Her car was crushed by falling debris.

“My bed moved, so it was a pretty strong explosion. I immediately went to the window to look outside, because I knew it came from the northern direction,” she said. “You couldn’t see anything, because it was smoke. So after about five minutes, I looked out, and I saw, one, my car was totaled; and then, two, I saw the building next door was completely gone.”

The two neighboring buildings were temporarily evacuated after the explosion, as a precaution.