Two pieces of mail tested positive for ricin by the Pentagon mail facility were addressed to Defense Secretary Jim Mattis and Chief of Naval Operations Adm. John Richardson, the Defense News reported Tuesday.

The FBI, which is leading the investigation, is expected to conduct a second test to confirm the initial finding on Tuesday.

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

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

The substance "is suspected to be ricin," said Pentagon spokesman Chris Sherwood.

Pentagon's mail facility is located in a separate building on the grounds of the Pentagon, instead of within the Pentagon building itself.

Ricin, a poison found in castor beans, can be used in powder, pellet, mist or acid form. If ingested, it causes nausea, vomiting, internal bleeding of the stomach and intestines and can be deadly.