ST. LOUIS COUNTY, Mo. - Three are hospitalized following an explosion Monday morning in unincorporated north St. Louis County.

About 20 homes were affected, and those at the Walmart on North Lindbergh and nearby businesses had to evacuate.

“I was with a customer talking to her, trying to figure out what she wanted and in the middle of our conversation we hear this loud explosion,” said Priska Lay. “I thought it was upstairs because it shook the whole building; it sounded like something on top of the roof.”

Residents nearby the incident told FOX 2 they felt the shock. Brian Jackson was at home when he heard the explosion.

“We saw a billow of smoke once we got down to the area,” Jackson said. “I went over to check and saw one house on fire and the other catching on fire. At the time, I had no idea the house that exploded was no longer.”

Rosemary Shelton told FOX 2 that she was working from home and in a meeting.

“I heard this tick-boom; before I could get up to see what it was, the window collapsed and fell in my lap,” Shelton described. “I was trying to push the window off me; the other window fell and pictures started falling off the wall. I thought there was a tornado going on.”

Investigators are still working to determine what caused the explosion, but there were no injuries reported at the local businesses impacted.