Senior Capitol Hill staff were warned that “multiple letters containing suspicious substances” were mailed out across the country and to multiple congressional offices on Thursday.

An email notice obtained by Fox News Digital sent to chiefs of staff, administrative managers and state directors said the U.S. Postal Inspection Service, the FBI and U.S. Capitol Police were “investigating multiple letters containing suspicious substances that were mailed to various addresses nationwide, including multiple Congressional offices.”

“Law enforcement officials are working to intercept any additional letters before they are delivered, however, it is possible that some letters may reach their destinations,” the notice from the U.S. Senate Sergeant at Arms read.

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Fox News Digital reached out to each agency investigating the matter for comment on the nature of the suspicious substances, how many letters were sent and retrieved and how many congressional offices could be affected. The notice was first reported by Punchbowl News.

Included in the notice was an image of one of the envelopes that included a return address in the top-left corner for the Democratic Socialists of America’s (DSA) headquarters in New York City. 

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Demonstrators carry signs from the Metro DC Democratic Socialists of America during a pro-Palestinian protest in Washington on Oct. 10, 2026. (Brooke Curto/Fox News Digital)

It is unclear whether the DSA had any involvement in sending the letters. Fox News Digital reached out to the DSA for comment but has not heard back.

The notice warned that “all suspicious letters, packages, or substances should be treated as potentially hazardous.”

Authorities have not publicly identified the substances or said whether they pose a danger.

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It comes on the 25-year anniversary when an anthrax-laced letter was opened by a staffer in then-Senate Majority Leader Tom Daschle’s office on Oct. 15, 2001.