WASHINGTON — The Capitol Police, FBI and U.S. Postal Inspection Service are investigating letters containing “suspicious substances” that were mailed to congressional offices and other addresses across the country, according to a memo sent to senior congressional staff on Thursday, Oct. 15, 2026. The memo from the Senate Sergeant at Arms says law enforcement is working to intercept additional letters before delivery, but warns that some could still reach their destinations. Investigators have not said whether any of the letters made it to Capitol Hill. The notice included a photograph of a suspicious envelope and advised staff to watch for warning signs, including messy handwriting and postmarks inconsistent with the return address. Authorities are also investigating reports of suspicious letters sent to lawmakers’ state and district offices. Election offices have received similar mail, according to a joint statement Wednesday from the National Association of Secretaries of State and the National Association of State Election Directors. The groups said they are coordinating with law enforcement to protect staff and facilities, with security measures and contingency plans in place while keeping election operations running. Congressional mail is routed through separate House and Senate screening facilities in Maryland. There, letters and packages are screened, irradiated and tested for a range of substances before being sent to the Capitol complex—a process that can delay delivery by as much as two weeks. The screening system was established after the 2001 anthrax attacks. It has also intercepted suspected explosive devices mailed to prominent Democrats in 2018 and a ricin-laced envelope sent to Sen. Roger Wicker, a Mississippi Republican, in 2013. Members’ state and district offices may also send mail through screening before delivery or use a similar security service. The investigation underscores the continuing security risks posed by threats sent through the mail, including to election personnel and public officials beyond Washington.