BELOIT, Wis. (AP) -

The Beloit police department has ended its investigation into an envelope dropped off at a health clinic that prompted dozens to be quarantined and sent 15 people to the hospital as a precaution.

Fire Chief Brad Liggett says no substance was found when officials tested the envelope, bill, check and the work area.

Police Capt. Vince Sciame says the man who dropped off his bill and payment Wednesday morning at the Beloit Area Community Health Center was forthcoming and says he didn't put anything in the envelope. Employees called 911 after three people who touched the envelope had burning in their eyes and nose.

Sciame says the investigation is considered closed pending more crime lab tests. He says it's possible they will never know what caused the symptoms.