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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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A U.S. Capitol Police officer at the Capitol in Washington, D.C., on Friday, Oct. 10, 2025. (Valerie Plesch/Bloomberg via Getty Images)
The notice was first reported by Punchbowl News.
An FBI spokesperson declined to confirm specific instances or letters, but said in a statement that, “The FBI and U.S. Postal Inspection Service are investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices.”
“Some of the letters contained an unknown substance, and we are working closely with our law enforcement partners to respond to each incident,” the spokesperson said. “We are also working with our partners to identify the individual or individuals responsible for the letters. This FBI and the U.S. Postal Inspection Service will continue to safeguard the public and secure elections for American citizens.”
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)
Included in the Senate Sergeant at Arms’ 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.
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.
