Ballot drop boxes were set on fire in the metro area of Portland, Oregon, resulting in hundreds of mail votes in the United States election being destroyed.

Just outside Portland in Vancouver, Washington, firefighters removed burning ballots from a drop box, according to video and photos from KATU-TV.

The footage showed firefighters stamping out a smouldering pile of ballots that appeared to have been reduced to ashes.

Clark County auditor Greg Kimsey, who administers elections in the area, said hundreds of ballots had been destroyed, but he did not know exactly how many.

Officials will sort through them so they can contact some of the affected voters, but he said there is no way to know who many of the ballots belonged to. Voters who dropped off their ballots after 11am on Saturday should contact his office to receive a new ballot, he said.

The fire in Vancouver erupted hours after an incendiary device sparked a fire in a ballot drop box outside the Multnomah County elections division in Portland, according to police and the elections division. Police released a photo showing the drop box with a charred hole.

Police quickly deployed a fire suppressant at about 3.30am that protected most of the ballots in that drop box, according to the elections division. Three ballots were damaged.

Election officials used unique identifiers to determine who the ballots belonged to and will contact the voters so they can vote again, the elections division said in a news release.

“Make no mistake, an attack on a ballot box is an attack on our democracy and completely unacceptable,” Oregon Secretary of State LaVonne Griffin-Valade (Democrat) said. “Whatever the motivation behind this incident, there is no justification for any attempt to disenfranchise voters.”

No other ballot drop boxes in Multnomah and Clark counties were damaged today, according to election officials. In Clark County, ballots will now be picked up daily, Kimsey said. In Multnomah County, drop boxes are monitored by security teams around the clock.

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Three weeks ago, police found a suspicious device near a different drop box in Vancouver after someone reported seeing smoke at about 4am, according to police. The drop box was not damaged in that incident.

Oregon and Washington conduct their voting largely by mail. Election officials send voters ballots, and they can return them by mail or at drop boxes.

The ballots that were destroyed in Vancouver inject a new element of uncertainty into one of the most closely watched congressional races in the country.

Democratic Representative Marie Gluesenkamp Perez faces a challenge from Republican Joe Kent, whom she beat by 1% two years ago in Washington’s third district. In social media posts, both candidates urged voters to ensure that their ballots were received by election officials.

Washington Secretary of State Steve Hobbs (Democrat) said a suspected incendiary device was placed in the ballot drop box in Vancouver. The FBI is among the law enforcement agencies that responded to the scene, he said.

“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,” Hobbs said.

He and other officials said voters should visit votewa.gov to track whether their ballot has been received by election officials.

The incidents occurred days after a 35-year-old man was arrested on suspicion of setting fire to the contents of a US Postal Service mailbox in Phoenix, Arizona, authorities have said. Ballots were among the items that were damaged during that fire.

The man, Dieter Klofkorn, allegedly admitted to starting the fire, and he told law enforcement that the fire was not tied to the election. About 20 ballots were affected, election officials said.

Officials will be able to process most of the affected ballots and will contact the voters for any they cannot, said Maricopa County recorder Stephen Richer.