Pittsfield Township firefighters visited the Burger King restaurant on Washtenaw Avenue twice more on Thursday and didn't find heightened levels of carbon monoxide, authorities said.

Fire Capt. Jeff Foulke said a monitor was used to test the air in the afternoon and evening, but didn't find anything unusual.

On Thursday morning, three employees at the restaurant at 4885 Washtenaw Ave. fell ill around 4:30 a.m.

Firefighters responded to the restaurant at about 8 a.m. after the hospital alerted the department about three people being treated there for elevated levels of carbon monoxide in their bloodstream.

The night manager drove himself to the hospital, and two other male employees went there in another car after the morning manager came to work and noticed they didn't look well.

Officials said they don't know what caused the employees to get sick.