Federal authorities are investigating letters with “suspicious substances” that have been mailed to congressional and election offices nationwide, including Maryland, and are urging recipients to treat the mailings as “potentially hazardous.”

The investigation by the FBI, U.S. Postal Inspection Service and U.S. Capitol Police was first disclosed in a message from the Senate Sergeant at Arms obtained Thursday by The Associated Press. The 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.”

In Congress, the threat appeared limited to the Senate, though the House was monitoring the situation, according to a person familiar with the matter who was not authorized to discuss the investigation publicly.

An FBI statement said the bureau and the Postal Inspection Service were “investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices.”

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“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident,” the statement read. “We are also working with our partners to identify the individual or individuals responsible for the letters.”

Mailed packages containing a powdery substance led to the evacuation this week of election offices in at least two states, Louisiana and Oklahoma. The Ohio secretary of state said the Postal Service intercepted a package containing a white powder intended for its office and noted that similar mailings had been sent to election officials in multiple states. Mississippi and South Dakota officials on Thursday said their election offices also were targeted by separate mailings but that the letters were intercepted by the Postal Service.

Maryland Elections Administrator Jared DeMarinis said he was notified by email that federal law enforcement agencies had intercepted mail addressed to the Maryland State Board of Elections in Annapolis.

“This madness needs to stop,” he said.

The mailings, he said, are meant to “harass and intimidate” election officials and silence voters’ voices.

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“They’re not going to deter me from doing my job in administering this election,” he said.

DeMarinis has been the defendant in lawsuits filed by the Trump administration trying to require states to hand over private voter data ahead of a contentious midterm election.

Authorities did not share with him anything about the package or its contents, De Marinis said. He expressed gratitude to law enforcement for keeping him and his staff safe.

News of the letters comes just three weeks before Election Day and is reminiscent of a similar threat in 2023, when election offices in six states received mailings that contained a white powder. In some cases, the incidents disrupted work around local elections.

Some of those letters contained threats and traces of the powerful opioid fentanyl, leading some election offices to stock up on the overdose reversal medication naloxone.

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The National Association of Secretaries of State and the National Association of State Election Directors issued a joint statement condemning what 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 said in their statement.

Deputy Postal Inspector Brendan Donahue told reporters Thursday that “any threat or attack on election officials is unacceptable” and that “we are also working with our partners on identifying the individual or individuals responsible for the letters.”

Associated Press writers Scott Bauer in Madison, Wisconsin; Sarah Rankin in Richmond, Virginia; and Nicholas Riccardi in Denver contributed to this report.

Baltimore Banner reporter Brenda Wintrode contributed to this article.