The Harris County Sheriff's Office said two men entered a Walmart on South Highway 6 in west Houston about 9:45 p.m. Sunday. The men sprayed a substance in the air and then fled the store. Employees and customers were forced to evacuate and three people close to the incident were hospitalized.
The Houston Chronicle said their condition remains unknown.
A Hazmat team entered and cleared the store of the hazardous substance but officials were unable to determine what it was.
Police have not released information about the suspects.