Federal authorities are investigating letters containing “suspicious substances” that were mailed to multiple congressional and election offices nationwide, law enforcement officials said Thursday.

The U.S. Postal Inspection Service, the FBI and the U.S. Capitol Police are probing the series of suspicious mailings, which were sent to secretary of state offices and other elections offices, U.S. Postal Inspection Service Deputy Chief Inspector Brendan Donahue confirmed to MS NOW.

“Some of the letters have contained an unknown substance, and we’re working closely with our law enforcement partners tmsno respond to each incident,” Donahue said, adding that authorities are in the process of identifying the source of the letters.

Messages sent to members of the Senate and House on Thursday morning informed lawmakers and their staff of the investigation.

The Office of the Sergeant at Arms of the House described the suspicious substance as a “white powder.”

No House offices have received the letters, according to the notice, and it was not immediately clear Thursday whether any of the mailings had reached Senate offices, nor to which offices they had been addressed.

“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 Senate message said.

The office of Sen. Chris Murphy, D-Conn., was among those that was sent a letter with a suspicious substance, according to spokesperson Josh Sorbe, but the letter was intercepted.

A letter was also sent to the office of Sen. Alex Padilla, D-Calif., spokesman Danny Rodriguez said. Padilla’s office was notified by the FBI about the letter, which was intercepted by USPS, according to Rodriguez.

Another letter had a return address that read “DSA,” from a suite in a New York City office building in the financial district of Manhattan, according to the notice sent to Senate offices.

The Democratic Socialists of America previously occupied the address, but have not been in the suite since 2013, and moved out of the building in January 2026, a DSA spokesperson said. The spokesperson added that the DSA was not involved.

In a statement to MS NOW, an FBI spokesperson confirmed the agency’s joint investigation with USPS, but did not confirm further details.

The suspicious letters come three weeks before Election Day. Both chambers are formally adjourned as members campaign for re-election in their districts.

CORRECTION (Oct. 15, 2026, 4:30 p.m. ET):
A previous version of this article incorrectly stated that Sen. Chris Murphy’s office received one of the letters mailed to congressional and election offices. A spokesperson for the senator’s office says one of the letters was addressed to him but was intercepted in the mail. The earlier story also incorrectly stated that another letter was sent to “DSA” at a Manhattan address. That was the return address.

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