Mail delivered to the Pentagon has tested positive for ricin, a potentially deadly poison, a Department of Defense spokesman told Business Insider on Tuesday.

The mail never actually entered the Pentagon but was delivered to a separate building — a mail processing system on its grounds.

The FBI is taking the lead on testing the mail and on conducting the overall investigation into where it came from.

“On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon’s remote screening facility,” Pentagon spokesman Col. Rob Manning told Business Insider. “The envelopes were taken by the FBI this morning for further analysis.”

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

At least two packages delivered to the Pentagon are suspected to contain ricin, and they were addressed to Secretary of Defense James Mattis and Chief of Naval Operations, Admiral John Richardson, CNN reports.

The Centers for Disease Control and Prevention (CDC) defines ricin as “a poison found naturally in castor beans.”

“If made into a partially purified material or refined into a terrorist or warfare agent, ricin could be used to expose people through the air, food, or water,” the CDC says on its website.

Ricin has been used in terror plots.