(DES MOINES, Nev.) -- A federal court building was locked down on Monday after a female employee opened an envelope and found white powder inside.

According to the Des Moines Register, the powder turned out to be simple acetaminophen, the pain reliever found in Tylenol. Police told the paper that the letter included a return address that corresponds with an existing U.S. Postal Service investigation.

The employee opened the letter just before 2 p.m. The office was sealed off for almost an hour.

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