'Suspicious' Letter with DSA-Linked Return Address Triggers FBI and Capitol Hill Investigation

Senior Capitol Hill staff were warned about suspicious letters containing unidentified substances that had been mailed to congressional offices across the country.

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Senior congressional staffers were alerted that "multiple letters containing suspicious material" were mailed across the country to several congressional offices on Thursday.

"Law enforcement officials are attempting to intercept any additional mail prior to delivery, but some mail may still reach its destination," the U.S. Senate Sergeant at Arms' notice said.

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The announcement was first reported by Punchbowl News.

An FBI spokesman declined to confirm the cases or specific letters, but said in a statement: "The FBI and the US Postal Inspection Service are investigating a series of suspicious letters sent to various addresses across the country, including the offices of the Secretary of State and their associated 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," a State Department spokesperson said. We are also working with our partners to identify the person or persons responsible for these letters. The FBI and the United States Postal Inspection Service will continue to protect the public and secure elections for American citizens."

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The Senate Sergeant at Arms section included a picture of one of the envelopes with a return address in the upper left corner of the Democratic Socialists of America (DSA) headquarters in New York City.

The warning warns that "all suspicious letters, packages or materials should be treated as potentially dangerous".

Officials have not publicly identified the substances or said whether they are dangerous.

On the 25th anniversary of when a letter containing anthrax was opened by a staff member in the office of then-Senate Majority Leader Tom Daschle on October 15, 2001.