A small bomb made with a bundle of sparkler fireworks exploded outside the Tacoma DOC field office at Center and J streets, according to Tacoma police.
No one was injured and no property was damaged, police spokeswoman Loretta Cool said.
It appears someone threw the explosive from a vehicle as it drove by, she said.
There are no suspects at this time and no threats were made to the office, Cool said.
The office was locked down while officers investigated, and reopened about 5 p.m.
Tacoma police are investigating, the Department of Corrections said.
The field office will be open Friday.