The FBI and the US Postal Department are investigating suspicious packages received by election officials in 17 states.
Federal investigators said they were collecting the packages and that some contained “an unknown substance”, though there were no reports of injuries.
They were sent to secretaries of state and state election officials across a swathe of the country from New York to Alaska.
It comes amid reports of rising threats directed at election officials across the US and warnings of political violence as November’s presidential election approaches.
The FBI and US Postal Department said they were trying to determine how many letters were sent and who was behind them, as well as their motive.
“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident and safely collect the letters,” the agencies said in a statement to BBC’s US partner CBS News.
The Associated Press news agency reported that packages were sent to election officials in Alaska, Georgia, Connecticut, Indiana, Kentucky, Massachusetts, Missouri, New York, Rhode Island, Iowa, Mississippi, Colorado, Kansas, Nebraska, Oklahoma, Tennessee and Wyoming.
Colorado Secretary of State Jena Griswold wrote in a post on X, formerly Twitter, that whoever posted the letters had called themselves as the “US Traitor Elimination Army”.
Officials in at least four of the states said there was no threat posed by the substances found in the packages. In Oklahoma, the Board of Elections said the substance was found to be flour.
In an interview with CBS, Michigan Secretary of State Jocelyn Benson, a Democrat, said her office was receiving threats daily through voicemails, emails, social media or in person, adding that “it’s escalating”.
This is not the first time suspicious mail was mailed to US election offices.
Last November, offices in Georgia, Nevada, California, Oregon and Washington were sent envelopes with fentanyl or other substances.
Federal investigators are separately questioning a gunman who was found lurking in bushes on a Florida golf course where Trump was putting on Sunday.
