Frank Harrill, the FBI supervisory senior resident agent in Spokane, announced that test result late Friday in an email.
Workers at a Spokane post office found the letter June 8 after it was returned as undeliverable. It had been addressed to the Central Intelligence Agency.
Harrill said the letter is similar in appearance to four other letters sent from Spokane last month that contained the deadly poison. Those were addressed to President Barack Obama, a federal judge, a Spokane post office and the nearby Fairchild Air Force Base.
No one has been sickened. Harrill said the fifth envelope was unopened and had not leaked.
A Spokane man named Matthew Ryan Buquet faces a one-count grand jury indictment for mailing threatening communications in connection with the case.
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