AVALON PARK, Fla. –  Thirteen people were treated for carbon monoxide poisoning after a 24-hour Walmart in central Florida was evacuated.

Orange County Fire Rescue officials say the gas had reached unsafe levels by 4 a.m. Sunday at the Avalon Park store. Fire officials had to wait three hours to enter the building because the CO levels were too high.

Fire Rescue spokeswoman Kathleen Kennedy say 13 people were treated for carbon monoxide poisoning. She says she believes only employees were treated. Seven people were taken to local hospitals and others refused treatment.

The cause is still under investigation, but constructions crews were working inside the store overnight with propane tools.

The Orlando Sentinel reports a hazmat team and mass casualty incident team responded. The building was deemed safe around 7:30 a.m.