Three members of staff who have been in contact with the powder have been placed in quarantine as a precaution.
Workers were called in to disinfect the entire premises, but this operation has now been completed.
The letter containing death threats was written in Dutch and included a call for foreigners to be expelled from the country immediately. The death threat was formulated as follows: "He will lose his head."
In the last government Mr Wathelet served as the Asylum and Migration Secretary.