The powder apparently came in a package from the US Postal Service itself—that is, USPS didn't just deliver the package, it was the return addressee.
A Hazmat team is inside City Hall right now, Banish says, but "the sample is so small they're having trouble testing what it is." Most City Hall employees are still in the building, "sheltering in place."
Update: Well, gee. Never mind. It's nothing. Really.