The Secret Service said it is investigating a suspicious letter addressed to President Trump that was intercepted Monday. Two packages addressed to top Pentagon officials were also found Monday and were suspected of containing the poison Ricin.

According to the Secret Service, the envelope addressed to the president never reached the White House. The agency did not say whether Ricin was present in the envelope.

"As a matter of practice, the Secret Service does not comment regarding matters of Protective Intelligence. However, in this instance, we can confirm that we are working jointly with our law enforcement partners to fully investigate this matter," the Secret Service said in a statement Tuesday evening.

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Ricin is a poison made from castor beans. The packages suspected of containing Ricin were found at a mailing facility on Monday, with one addressed to Defense Secretary James Mattis and the other to the Navy's top officer, Adm. John Richardson. Neither package entered the building.

"All threats directed towards the President, or any Secret Service protectee, are treated seriously and fully investigated," the Secret Service said in a statement Tuesday.