(CNN) Two pieces of mail delivered to the Pentagon mail facility on Monday have initially tested positive for ricin, according to a US defense official.

The two suspicious envelopes were addressed to Secretary of Defense James Mattis and to Chief of Naval Operations, Admiral John Richardson, the official told CNN.

The mail facility is in a separate building on the grounds of the Pentagon and the piece of mail which tested positive never entered the Pentagon building.

All US Postal Service mail received at the Pentagon mail screening facility on Monday is currently under quarantine and poses no threat to Pentagon personnel, according to Pentagon spokesperson Col. Rob Manning.

"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Manning said in a statement.

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