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.

Workers in Washington will begin searching through the damaged ballots for voter information in order to contact people about getting a new ballot, said Greg Kimsey, the Clark county auditor. Workers should be able to collect information from the ballots despite the damage, he said.

The attacks are under investigation by the FBI as well as state and local officials.

In surveillance images captured in Portland just before security personnel nearby discovered a fire inside the box, a Volvo can be seen pulling up to the drop box, said Mike Benner, a spokesperson for the Portland police bureau. The incendiary devices were attached to the outside of the boxes.

The fire at the drop box in Portland was extinguished quickly thanks to a suppression system inside the box and a nearby security guard, police said. The ballot box in Vancouver also had a fire suppression system inside, but that failed to prevent hundreds of ballots from burning, according to Kimsey.

The US attorney, Tessa M Gorman, and Greg Austin, the acting special agent in charge of the FBI Seattle field office, said in a joint statement on Tuesday that they were working together to investigate the fires and will hold the person or people responsible “fully accountable”.

No arrests had been announced as of Tuesday evening.

Several hours after the fire in Portland, another fire was discovered at a transit center drop box across the Columbia River in Vancouver.

Officials said that enough material from the incendiary devices was recovered to link the two fires on Monday, as well as an incident on 8 October when an incendiary device was placed at a different ballot drop box in Vancouver. No ballots were damaged in that incident.

In a New York Times report on Tuesday, two unnamed law enforcement officials told the newspaper that authorities had recovered devices at the scenes that had the words “Free Gaza”. But officials are investigating whether the attacker was a pro-Palestinian activist or using the phrase to create “discord”, the officials said.

Vancouver is the biggest city in Washington’s third congressional district, the site of what is expected to be one of the closest US House races in the country, between the first-term Democratic representative Marie Gluesenkamp Perez and Republican challenger Joe Kent.

Kimsey, the Clark county auditor, has urged voters who dropped their ballots in the transit center box after 11am 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, 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 would be checked more often up until election day.