FBI is investigating and workers are contacting voters about new ballots after incendiary devices were put in drop box

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Officials in south-west Washington were able to salvage almost 500 damaged ballots from a ballot box that was set on fire on Monday in what officials have called an attack on democracy ahead of a contentious US election.

An unknown number of ballots were destroyed when someone placed incendiary devices in a drop box in Vancouver, Washington, while three ballots were damaged in a fire at a box in nearby Portland, Oregon. Those fires and one other are linked, officials have said.