Two suspicious envelopes mailed to the Pentagon on Monday tested positive for ricin, prompting a shutdown of a mail processing facility that is separate from the Pentagon itself.

"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility. The envelopes were taken by the FBI this morning for further analysis," said Col. Rob Manning, a Pentagon spokesman.

"All USPS mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel,” he added.

One of the envelopes was reportedly address to Defense Secretary Jim Mattis and the other to Chief of Naval Operations Adm. John Richardson, according to CNN.

Mattis is currently traveling in Europe attending a NATO defense ministerial, which begins tomorrow in Brussels.

Ricin is a deadly toxin that is naturally occurring in castor beans.

In the wake of the Sept. 11 attacks, the Pentagon built a massive underground remote processing facility on the west side of the building to screen all deliveries, including mail, into the building.

No one is reported to be suffering any ill effects from exposure to the suspicious substance.