The F.B.I., Postal Service and Capitol Police are investigating a series of letters “containing suspicious substances” that were sent to senators, state election officials and their offices across the country in recent days, according to an email sent to Senate staff on Thursday.
The F.B.I. said in a statement that it was working to find those responsible for the letters but provided no information on potential suspects. Senate officials informed aides on Capitol Hill that the letters had tested negative for hazardous substances. The email to Senate staff, which was sent by the chamber’s top security officials, contained a picture of one of the envelopes that listed a return address for “DSA” in Manhattan that was previously the office of the Democratic Socialists of America.
Priscilla Yeverino, a spokeswoman for the D.S.A., said the group had not sent the letters. She added that it had not occupied the office suite listed since 2013 and had moved out of the office building entirely in January.
“D.S.A. did not send these letters,” Ms. Yeverino said.
It is unclear whether any of the letters reached the Capitol or any congressional office; mail for members of Congress is typically screened at a facility off-site and tested for hazardous materials. But spokesmen for two Democrats, Senators Christopher S. Murphy of Connecticut and Alex Padilla of California, said the senators had been notified by the F.B.I. that they were sent letters with suspicious substances.
In a separate notification to senior Senate staff, the office of the Senate’s sergeant-at-arms said that senators in both parties had been mailed letters to Capitol office buildings and state offices, according to two people who viewed the notice but requested anonymity to discuss a sensitive security issue.
The sergeant-at-arms linked the letters to similar ones that had been sent to secretary of state offices nationwide.
Secretaries of state in Ohio, Louisiana and Oklahoma have said that their offices had been sent mail that contained suspicious substances.
Frank LaRose, the Ohio secretary of state, said in a statement on Wednesday that the item sent to his office “contained a powdery white substance” but that Postal Service investigators intercepted it.
The letters come with just weeks until the midterm elections, as election officials have reported a growing number of threats and intimidating behavior.
In a joint statement, the National Association of Secretaries of State and the National Association of State Election Directors said they were “deeply concerned by these incidents.”
The incident was reminiscent of the anonymous mailing of powder-filled envelopes to state and local elections offices in 2023. Some of those letters appeared to have been laced with fentanyl, law enforcement officials said at the time.
“As we have each stated in the past, threats against election officials and workers have no place in our democracy,” the election officials’ associations wrote. “Those who administer our elections should never fear for their safety while carrying out their essential duties.”
Glenn Thrush contributed reporting.