The Mercer County Courthouse in Trenton was closed Thursday to test for traces of the potentially deadly Legionnaires' disease.

Workers searched the ducts of the building on S. Broad Street for the bacterium that causes the respiratory ailment. Officials say a researcher who often visits the courthouse, and was last there Friday, tested positive for the disease. The county executive says the woman is now in serious condition at the hospital.

Officials urge anyone who has been in the courthouse over the past two weeks to be on the alert for flu-like symptoms. The building is not expected to reopen until after Monday.