The FBI is investigating after two ballot drop boxes went up in flames in the Portland, Oregon, area in the early hours of Monday.
Hundreds of ballots were burned in a fire that started outside a park and ride station in Vancouver, a small city in Washington state less than 10 miles (16km) from Portland.
Local media reported that a device was stuck to the outside of a ballot drop box.
Meanwhile in the south-east part of Portland itself, investigators said, an incendiary device was placed inside a ballot box, causing a small fire.
Dedicated ballot drop boxes are used in a number of US states and cities to allow voters to submit their ballots early without having to wait in a queue on Election Day.
Ballots were last picked up from the Vancouver box on Saturday morning, and local officials said those who had deposited their ballots since then should contact the local elections office.
Videos showed workers extinguishing the fire and a large stack of smouldering papers.
Authorities said three ballots were damaged in the Portland fire, and that they were attempting to contact the voters who cast them so that they can be issued replacements.
Oregon and Washington are safe Democratic states in the presidential race, but the contest for a congressional seat is close in Vancouver – not to be confused with the much larger Canadian city.
Washington’s 3rd District includes Vancouver and a huge and largely rural section of the southern part of the state.
The seat is currently held by Democrat Marie Gluesenkamp Perez who in 2022 eked out a victory over Republican Joe Kent by less than 1% of the vote.
Gluesenkamp Perez and Kent are headed to a rematch in next week’s election.
There was no word from police on who might have planted the devices that caused the ballot-box fires. The BBC has contacted authorities in both cities for comment.
The chief elections official in Washington, Secretary of State Steven Hobbs, said in a statement: “We take the safety of our election workers seriously and will not tolerate threats or acts of violence that seek to undermine the democratic process.”
Last week a man in Phoenix, Arizona, was charged with setting a US post box on fire that contained around 20 ballots, court records show.
By contrast, the boxes set alight in Portland and Vancouver were only used for ballots.
In September, the US Department of Homeland Security warned that some social media users have been promoting the destruction and sabotage of ballot drop boxes ahead of the election.
With reporting by Sam Cabral and Merlyn Thomas
