Police believe three suspected arsons at ballot drop boxes in the northwestern US states of Washington and Oregon are linked, and have identified what they believe is a suspect vehicle in Oregon, officials said on Monday.

Two separate blazes erupted Monday in Portland, Oregon, damaging three ballots, and Vancouver, Washington, destroying hundreds, officials said. They said the two blazes resembled a third fire at a Vancouver ballot box on Oct 8 that did not succeed in damaging any ballots.

"We don't know the motive," Amanda McMillan, assistant chief of the Portland Police Department, told a news conference on Monday. "But we do know that acts like this are targeted and intentional."

Potential charges for the suspect include possession of a destructive device and illegal acts related to voting machines, among others, a Portland Police official said.

Footage from Vancouver news outlet KATU on Monday showed smoke pouring out of a ballot box in that city with multiple ballots appearing to be in flames. Local officials said voters who put their ballots in the box after 3:30 p.m. PDT (2230 GMT) on Saturday should contact election officials.

The Vancouver Police Department said it responded to an arson attack at 4am PDT (1100 GMT) on Monday and removed a "suspicious device" located next to the ballot box.

Earlier on Monday morning, police responded to a similar incident at a ballot box in Portland, Oregon, just across the Columbia River, caused by an incendiary device. Multnomah County's election director said only three ballots were damaged in that fire, and that those voters will be contacted to fill out replacement ballots.

Fire suppressant installed inside the boxes mitigated the damage in Portland and the first Vancouver fire, but did not function in Monday's fire in Vancouver.

The Washington secretary of state's office urged voters to check the status of their ballots online and to request a replacement if theirs was unaccounted for.

"I strongly denounce any acts of terror that aim to disrupt lawful and fair elections in Washington state," Secretary of State Steve Hobbs said in a statement. "Despite this incident, I have complete confidence in our county elections officials’ ability to keep Washington’s elections safe and secure for all voters."

The Vancouver boxes were located in the state's 3rd District, site of one of the most competitive U.S. House of Representatives races, featuring Democratic Representative Marie Gluesenkamp Perez and Republican challenger Joe Kent.

Gluesenkamp Perez called for overnight law enforcement monitoring of ballot boxes in the district.

The 36-year-old car mechanic won her race in 2022 by fewer than 3,000 votes.

In Phoenix, a man was arrested after a mailbox there was set ablaze on Thursday. About 20 ballots were destroyed, according to local media reports.