The FBI and the U.S. Postal Inspection Service (USPIS) are investigating a series of suspicious mailings sent to state election officials and congressional offices in multiple states, authorities confirmed Thursday.

Some of the letters contained a white powdery substance. Officials described the mailings as "potentially hazardous" and said they are working to identify those responsible.

"We are investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices," the FBI 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."

Authorities declined to specify how many offices had been targeted. Among those affected were the Oklahoma State Election Board, the Louisiana Secretary of State's office, and the Ohio Secretary of State's office.

The Louisiana office was evacuated after a staff member opened one of the letters on Tuesday. The employee was taken to the emergency room as a precaution but was not injured. Law enforcement found no evidence of harmful substances, and the office has since reopened.

Ohio Secretary of State Frank LaRose said his office was the intended recipient of a letter containing white powder, but the USPIS intercepted it before delivery.

Senator Chris Murphy, a Connecticut Democrat, also received a letter containing a suspicious substance, though it was intercepted before reaching his office, according to his spokesman.

Suspicious packages were also reported at the New York State Capitol in Albany and the Wyoming State Capitol in Cheyenne.

Deputy Chief Inspector Brendan Donahue of the USPIS confirmed that authorities were able to stop the delivery of at least one of the mailings. "Some of the letters have contained an unknown substance, and we're working closely with our law enforcement partners to respond to each incident," Donahue said.

A communication from the Senate's Sergeant at Arms to congressional staff warned that 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 note advised staff to treat all suspicious letters, packages, or substances as "potentially hazardous" and to handle them in accordance with established safety protocols. Staff were warned not to handle suspicious items and to leave the area where they are discovered. The communication noted that suspicious mail could be identified by discoloration, powder emanating from the envelope, and postmarks that do not match the return address. An image included in the communication showed a return address labeled "DSA" with a New York City return address.

"The FBI and the U.S. Postal Inspection Service will continue to safeguard the public and secure elections for American citizens," the agencies said in a joint statement.

The incidents come less than three weeks before the midterm elections on November 3, which will determine the balance of power in Congress.

The investigation echoes similar events ahead of the 2024 election, when suspicious mailings were sent to more than 15 state election offices. A year before that, suspicious envelopes, some laced with fentanyl, were sent to election offices in at least five states.

The National Association of Secretaries of State and the National Association of State Election Directors issued a joint statement saying that "those who administer our elections should never fear for their safety while carrying out their essential duties."