Crews were called early Wednesday morning to the Wells Fargo Plaza, located in the 400 block of Market Street.
It was around 4:45 a.m. when four employees working in the mail room discovered the powder inside a mail sack.
The fire department was called, and upon arriving on the scene crews suited up and isolated the four employees as a precaution.
The mail room and loading dock were isolated and access was restricted to the building while a series of tests were conducted on the mystery substance.
The scene was declared under control around 6:22 a.m.. The Battalion Chief on the scene tells Action News that the powder was not dangerous. However, there is no word yet on the nature of the substance.
Officials shut down a section of Fourth Street between Arch and Market Streets during the investigation, but the scene has since cleared.