At least two packages mailed to the Pentagon this week are believed to contain the poison ricin, and the FBI is now investigating them, the Pentagon said Tuesday.

The packages were addressed to Defense Secretary Jim Mattis and Chief of Naval Operations Adm. John Richardson, a defense official said on the condition of not being named. The packages triggered alarms Monday as they were undergoing security screening at a mail processing center that is on the Pentagon campus, but not inside the building.

“It’s suspected to be ricin,” said Pentagon spokesman Chris Sherwood.

Department of Defense employees inside the Pentagon's mail screening facility inspect envelopes and packages, one step in the Pentagon's mail screening process, before the mail is released to recipients on Monday, Oct. 1. (Tara Copp/Military Times)

The envelopes were taken by the FBI this morning for further analysis, said Pentagon spokesman Army Col. Rob Manning. The Pentagon mail that arrived with the packages Monday is now on hold.

The Associated Press reported that a third suspicious envelope was sent to President Donald Trump, but it was not received at the White House. The Secret Service didn’t disclose any details about what was in the envelope or where it was received, and the White House had no comment, according to AP.

Later Tuesday, the Pentagon allowed reporters to view the facility where every piece of inbound mail is inspected and screened before it is allowed to move on to its intended recipient. Inside, more than two dozen Department of Defense employees work 24 hours a day, screening the more than 500,000 pieces of mail sent to the Pentagon each year, a DoD official said on the condition of anonymity. In addition to the employees' physical screen, there are additional tests that the official would not elaborate on.

The official did not want to say whether the facility had ever previously had a positive result like Monday’s ricin results, but indicated that it was extremely rare.

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The screeners undergo regular medical tests and wear protective suits while working, due to the nature of their work, the official said.

Department of Defense employees inside the Pentagon's mail screening facility inspect envelopes and packages on Monday, Oct. 1. (Tara Copp/Military Times)

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

Ricin is a naturally occurring poison that can be made from castor beans and can be turned into a powder.