Officials at the state prison in Lycoming County said a white substance was found Wednesday morning while employees were sorting the mail.
Prison officials said a postal inspector was called in to investigate and determined it was lactose, a sugar that is found in milk.
At first, no one knew that and five prison employees in that mail room were taken to the hospital as a precaution.
Emergency crews rushed to the State Correctional Institution at Muncy along Route 405. Officials said a little before 11 a.m. employees discovered something potentially dangerous while sorting the mail.
"We had an apparent white substance that revealed itself in the mail room. Once the mail room supervisor realized what it was, we secured the area," said Troy Edwards, a Corrections Superintendent Assistant at SCI Muncy.
The prison is one of only two all-female prisons in the state. Edwards said the prison outside Muncy houses roughly 1,500 inmates, has several hundred employees working at any one time, and needless to say, gets lots of mail.
"Everyone responded appropriately, a fantastic response given a potential dangerous situation," added Edwards.
The envelope was intended for an inmate and had a return address, according to state prison officials. The FBI is now handling the investigation.
All five mail room employees were released from the hospital with clean bills of health, said officials at the prison.
The investigation continues into who sent the letter containing the powder and why.