The source of an irritant that sent three mail center workers at a southwestern Illinois air base to the hospital may forever be a mystery.
A Scott Air Force Base spokesman says authorities were unable to pinpoint what caused Wednesday's illnesses and a brief evacuation of that portion of the installation. The spokesman says the investigation is now closed.
Officials have said it's possible whatever caused the mail handlers to develop respiratory or skin reactions may have dissipated by the time tests were done.
Thirteen other people were decontaminated Wednesday as a precaution.
Lt. Benjamin Garland says the mail center remains closed, although the rest of the base is open and considered safe.
He says there's no evidence the questioned mailing was terrorist-related.