Authorities in Portland, Oregon, and Vancouver, Washington, are investigating two separate incidents in which ballot drop boxes were set on fire on Monday, destroying hundreds of ballots as voters cast early votes ahead of next Tuesday’s presidential election.

Police believe the fires, both involving incendiary devices, were coordinated, with a similar vehicle seen at each scene, according to the Associated Press.

They are linking the recent incidents to an attack on a ballot box in Vancouver on 8 October, in which an incendiary device was also used.

The attacks came amid heightened election tensions, as early voting reached nearly 42 million ballots cast nationwide.

Oregon and Washington, both vote-by-mail states, rely heavily on ballot drop boxes to facilitate the voting process.

Steve Hobbs strongly condemned the attacks, calling them “acts of terror” intended to disrupt the democratic process.

“I strongly denounce any acts of terror that aim to disrupt lawful and fair elections in Washington state,” Hobbs said.

Voters were urged to check the status of their ballots online to confirm they have been received.

If a ballot is marked as “not received,” voters can print a replacement or visit their local elections office to ensure their vote is counted.

Washington and Oregon send ballots by mail to all registered voters a few weeks before election day, with return options that include mailing them back or placing them in drop boxes.