Crews were called around 4:45 a.m. Wednesday to the Wells Fargo Plaza located in the 400 block of Market Street.
Investigators tell Action News that a white powdery substance was found in the mail room.
The scene was declared under control around 6:22 a.m. while investigators continue to sort out the details.
The white powder has not been identified but tests are underway to determine the nature of the substance.
Officials have shut down a section of Fourth Street between Arch and Market Streets.
So far, there have been no evacuations.