The FBI, U.S. Postal Inspection Service and U.S. Capitol Police are investigating a series of suspicious mailings sent to congressional offices and election officials nationwide, warning recipients that some letters contain unknown substances and should be treated as potentially hazardous.

The investigation was disclosed in a notice from the Senate Sergeant at Arms obtained Thursday by The Associated Press. The message informed Senate offices that law enforcement agencies were working to intercept additional letters, while cautioning that some could still reach their intended destinations. The threat appeared to be limited to the Senate within Congress, although the House was monitoring the situation, according to a person familiar with the investigation.

In a statement, the FBI confirmed that suspicious mailings had been sent to addresses across the country, including offices belonging to state secretaries of state and associated election agencies.

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident,” the bureau said, adding that investigators were working to identify those responsible.

The investigation follows several incidents involving packages containing powdery substances. Election offices in Louisiana and Oklahoma were evacuated this week after receiving suspicious packages. Ohio's secretary of state also reported that the Postal Service intercepted a package containing white powder intended for the office, noting that similar mailings had been directed at election officials in multiple states.

The incidents have raised concerns about election security as officials prepare for the November 3 midterm elections, which will determine control of Congress and feature contests for numerous state and local offices.

The situation also recalls a series of suspicious mailings in 2023, when election offices in six states received letters containing white powder. Some of those mailings included threats and traces of fentanyl, a powerful synthetic opioid, disrupting election-related work in some locations. Several offices responded by obtaining naloxone, a medication used to reverse opioid overdoses.

The National Association of Secretaries of State and the National Association of State Election Directors condemned the threats in a joint statement, emphasizing the importance of protecting election workers.

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

Deputy Postal Inspector Brendan Donahue also condemned the incidents Thursday, saying, “Any threat or attack on election officials is unacceptable.” He said postal inspectors were working with law enforcement partners to identify those responsible.

Authorities have not publicly identified a suspect or disclosed the substances involved in the latest mailings. The investigation remains ongoing as federal agencies work to respond to reported incidents and prevent additional letters from reaching election and government offices.