The mail room at the Twin Rivers Unit was emptied and sealed off while officials investigated, said Selena Davis, a spokeswoman for the state Department of Corrections.
Firefighters retrieved a sample of the powder and tested it on site.
The initial testing revealed that the powder likely came from a fire extinguisher. It wasn't clear Monday how the mail was contaminated.
Firefighters contacted the state Department of Ecology to dispose of the substance.
No injuries were reported.
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