In tweets posted by the police department, HPD stated administrative personnel at its building on 33 Artesian Place opened the envelope addressed to the department late in the morning.
Houston Fire Department hazmat crews were called, and their early tests indicated the powder as the narcotic.
Four HPD employees who were present when the envelope was opened were checked out for safety reasons.
According to HPD, the envelope had a name and a return address, as well as a piece of paper containing a few names on it.
Police believe the envelope was an attempt to provide names of people selling the drug and that it wasn't intended as a threat to law enforcement.
An investigation is ongoing.