BELLEVILLE, Wis. -- People at a Belleville business were transported to a hospital on Wednesday night after falling ill.

About 10 people were being treated for carbon monoxide poisoning after they became ill at the Family Dollar Store in the 1000 block of River Street in Belleville, WISC-TV reported.

Emergency responders were called to the store on two occasions Wednesday. The Belleville Fire Department was called to the store at 4 p.m. and then again around 6 p.m. Wednesday.

Rescue crews transported two people to Madison hospitals after the first call. More than nine people were transported to hospitals on the second call. Authorities said all the people showed classic symptoms of carbon monoxide poisoning, such as nausea and headache.

However, firefighters said they couldn't find a source for the carbon monoxide and registered very low readings using carbon monoxide sensors.

"We have all the modern carbon monoxide testers, but everything showed, if anything at all, a real low-grade reading. So, it's just very odd," said Ron Babler, deputy chief at the Belleville Fire Department.

The store can't reopen until authorities determine what caused the victims, all employees, to become sick.

Station 6, the Hazmat unit from Madison, was called in Wednesday night to investigate.

Stay tuned to WISC-TV and Channel 3000 for continuing coverage.