It all began around 10:30 a.m. on Friday when officials at the House of Correction at 8001 State Road in the Holmesburg section.
Police say a corrections officer was examining mail and found a piece with a yellow, powdery substance in it.
She informed her supervisor, and, following protocol, the Philadelphia Fire Department was called.
The substance turned out to be cake mix.
Prison spokesman Bob Eskind tells Action News that the letter was originally sent from the prison and then returned because of a lack of postage.
The Administration Building, which houses the Warden's Office, Deputy Warden's office and administration support staff was evacuated after the powder was found.
The incident was cleared shortly after 12:30 p.m.