Crews were called around 5:30 a.m. Thursday to the 5900 block of North Broad Street near Champlost.
An unknown substance was discovered inside an envelope in the building where Northwest detectives and 35th district officers work.
Police say the envelope was part of evidence and was not mailed to the station.
The second floor of the buidling was evacuated as a precaution.
Once crews were on the scene, they tested the white powdery substance. Preliminary testing showed the substance to be inert.
No injuries were reported.
Operations were back to normal by 7:00 a.m.