The FBI and Postal Service are investigating letters containing “suspicious substances” that were sent to congressional and election offices across the United States.

Authorities are urging recipients to treat the letters as “potentially hazardous” as they investigate an unknown substance found inside some of the pieces of mail.

A notice sent to Senate offices said that law enforcement officials were working to intercept any additional letters, but “it is possible that some letters may reach their destinations.”

“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,” an FBI spokesperson said in a statement.

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident. We are also working with our partners to identify the individual or individuals responsible for the letters. This FBI will continue to safeguard the public and secure elections for American citizens.”

In Congress, the situation appeared isolated to the Senate, but the House was monitoring, a person familiar with the matter told The Associated Press.

Meanwhile, packages containing a powdery substance triggered the evacuation of election offices in at least two states, Louisiana and Oklahoma. The Postal Service intercepted a package containing a white powder intended for the Ohio secretary of state, who said similar mailings had been sent to election officials in multiple states.

The warning over the suspicious letters comes three weeks before Election Day. A similar threat caused alarm in 2023, when several election offices were sent envelopes containing the powerful opioid fentanyl.

The National Association of Secretaries of State and the National Association of State Election Directors issued a statement condemning the letters sent this week, which they said were threats against election staff.

“Those who administer our elections should never fear for their safety while carrying out their essential duties,” they wrote.

More to follow…