CARLISLE, Pa. >> State police and federal authorities are investigating after a letter sent to a Pennsylvania police barracks containing what police call a “biohazard” sickened three employees.

Police say two state troopers and a civilian staff member became ill immediately after one of the troopers opened the letter shortly after 2 p.m. Thursday at the Carlisle barracks.

The barracks was evacuated until about 7:30 p.m. Thursday. The three employees were discharged from the hospital later Thursday and returned to work Friday. The FBI is examining the letter and envelope.

Trooper Brent Miller said Friday the investigation has “ruled out all serious biohazards” but he could provide no further information on what the letter contained. He said the letter had been addressed to the trooper who opened it.