About 475 damaged ballots were retrieved from a ballot box that was burned in southwest Washington, a county official said on Tuesday.
Clark County auditor Greg Kimsey said workers will begin searching through the damaged ballots for voter information in order to contact them about getting a new ballot.
He said officials believe that although damaged, the workers will be able to pull voter information from the ballots.
The damaged ballots are separate from an unknown number that were destroyed, he said.
Authorities have said that enough material from the incendiary devices was recovered to link the two fires on Monday, as well as an October 8 incident, when an incendiary device was placed at a different ballot drop box in Vancouver. No ballots were damaged in that incident.
Surveillance images captured a Volvo pulling up to the drop box in Portland just before security personnel nearby discovered a fire inside the box, Portland Police Bureau spokesperson Mike Benner has said. The incendiary devices were attached to the outside of the boxes.
US attorney Tessa M Gorman and Greg Austin, acting special agent in charge of the FBI Seattle field office, said in a joint statement on Tuesday that they wanted to assure residents that they are working together to investigate the fires and will work to hold the person or people responsible “fully accountable”.
No arrests had been announced on Tuesday evening.
Several hours later, another fire was discovered at a transit centre drop box across the Columbia River in Vancouver, the biggest city in Washington’s 3rd Congressional District, the site of what is expected to be one of the closest US House races in the country, between first-term Democratic Representative Marie Gluesenkamp Perez and Republican challenger Joe Kent.
The ballot box in Vancouver also had a fire suppression system inside, but that failed to prevent hundreds of ballots from burning, according to Mr Kimsey. He has urged voters who dropped their ballots in the transit centre box after 11am on Saturday to contact his office for a replacement ballot.
The office is increasing how frequently it collects ballots and changing collection times to the evening, Mr Kimsey said, to keep the ballot boxes from remaining full of ballots overnight when similar crimes are considered more likely to occur.
Officials in at least two other counties in Washington — including in King County, where Seattle is located, — announced on Tuesday that ballot drop boxes will be checked more often up until Election Day.
