Officials suspect a prison inmate in another state sent a letter containing white powder to a federal court building in Des Moines.
Postal inspector Jim O'Hara said the inmate already is being investigated for allegedly sending a similar threatening letter with white powder to another government building in Iowa.
O'Hara would not release the inmate's name or the state where the inmate is imprisoned.
The federal court annex in Des Moines was locked down for nearly an hour Monday after the letter was opened. The building was reopened after a test showed the powder was acetaminophen, a common over-the-counter pain reliever