About 2,500 employees returned to work last week at a Ford Motor Co. engine parts plant near Cleveland. It was shut down for five days due to a deadly outbreak of Legionnaires’ disease.

The source of the disease still is not known, although some suspect the plant’s cooling towers. "We do not know that it was the cooling towers," says Della DiPietro, a Detroit-based Ford spokesperson. Regular maintenance is performed on the towers two times each year, she says.