The Secret Service has confirmed that President Donald Trump also received an envelope containing the poison ricin after the Pentagon discovered two suspicious envelopes that tested positive for the deadly substance.

According to the Secret Service, the envelope was addressed to the President on Oct. 1 but never made it to the White House.

“As a matter of practice, the Secret Service does not comment regarding matters of Protective Intelligence,” the Secret Service said in a statement. “However, in this instance, we can confirm that we are working jointly with our law enforcement partners to fully investigate this matter. Further, all threats directed towards the President, or any Secret Service protectee, are treated seriously and fully investigated.”

The other two envelopes discovered by mail screeners at the Pentagon were addressed to Defense Secretary James Mattis and to Chief of Naval Operations Admiral John Richardson, CNN reported. The envelopes have been turned over to the FBI and all U.S. Postal Service mail received at the screening facility “is currently under quarantine,” according to Christopher Sherwood, a Pentagon spokesman.

Ricin is 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 exposed to people through the air, food or water, according to the CDC.

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Write to Gina Martinez at gina.martinez@time.com.