The three-story building near 58th Street and Calumet in the Washington Park neighborhood collapsed around 7 p.m. Sunday.
PHOTOS: Washington Park apartment building collapses
Two female residents and a dog were rescued. The women were taken to the hospital in serious to critical condition.
Meanwhile, the fire department is working on a cause to determine if there was an explosion or if the building just collapsed.
As firefighters thoroughly search the rubble to figure out how the apartment building could go from standing to being completed leveled within seconds, neighbors recall what they saw and heard.
"When I opened up the back door, I saw a big flash. It was very colorful and it shot out from the side of the building," said neighbor Tryus Lester. "When it shot out from the side of the building, it was a big boom right after that."
"I heard a loud, sonic boom, the lights went out and my whole house lit up on the inside, and I stay on the third floor," neighbor Romell Collins said. "When I got up and walked out to see what was going on, it looked like 9-1-1 outside.
Angela Brooks lives right next door and her bedroom window faces the leveled building.
"It all happened really quick," she said. "I probably had been home a half an hour. The building was standing when I came in the house, and the next time I came out, it's what you see now," she said.
Both women trapped in the rubble were rescued quickly and taken to area hospitals.
"They're both in serious to critical condition, but they're both talking," said Chief of Special Operations Michael Fox, Chicago Fire Department. "From what we gather from them, those were the only two people, beside the dog, that were in the building. We will pull it apart, though, and search, just to make sure."
The search also includes looking for a cause.
"We're not sure if there was an explosion or just a collapse. As we dig through we'll be able to find out more what caused it," Chief Fox said.
The building itself is about three years old. While there are four units inside, there were only two occupants.
The fire department said they will stay on the scene and thoroughly go through the rubble until they come up with a cause.