The suspected arsonist behind a spate of ballot drop box fires could be planning more attacks in the run-up to the presidential election, police warned.
It comes after ballot boxes were set alight in Portland, Oregon, and Vancouver, Washington.
Police said on Wednesday the assailant sparked the blazes using incendiary devices marked with the messages 'Free Palestine' and 'Free Gaza '.
They believe he had a 'wealth of experience' in metalworking and welding, Portland Police Bureau spokesman Mike Benner said.
Authorities described the suspect as a white man, age 30 to 40, who is balding or has very short hair.
Authorities seen working to put out a fire at a ballot box in Vancouver, WA started early Monday morning. It was one of two fires set at two ballot boxes in two separate states
Police previously said surveillance video showed the man driving a black or dark-colored 2001 to 2004 Volvo S-60.
The vehicle did not have a front license plate, but it did have a rear plate with unknown letters or numbers.
Investigators are trying to identify the person responsible and the motive for the suspected arson attacks, which destroyed hundreds of ballots at one location in Vancouver, Washington on Monday.
The drop box's fire suppression system did not work as intended.
Greg Kimsey, the elected auditor in Clark County, Washington, which includes Vancouver, said the exact number of destroyed ballots was not known, and that about 475 damaged ballots had been retrieved from the box.
Authorities are trying to figure out whether the person who left the devices actually had pro-Palestinian views or used the message to try to create confusion.
Investigators are seeking a suspect vehicle, a black Volvo S-60, that was captured on security footage leaving the devices before they exploded at the ballot boxes
Surveillance images captured the Volvo pulling up to a drop box in Portland just before security personnel nearby discovered a fire inside the box on Monday, according to Benner.
The early-morning fire at the drop box in Portland was extinguished quickly thanks to a suppression system inside the box as well as a nearby security guard, police said.
Only three of the ballots inside were damaged.
Election staff on Wednesday planned to sort through the damaged ballots for information about who cast them, in the hopes that those voters can be given replacement ballots.
An image released by the Portland Police Bureau shows the ballot box after an 'incendiary device' was discovered inside the box starting a fire
The ballot boxes were set aflame by an unknown arsonist in the Pacific Northwest on Monday
Kimsey urged voters who dropped their ballots in the transit center box between Saturday morning and early Monday to contact his office for a replacement ballot.
Authorities in Portland said Monday that enough material from the incendiary devices was recovered to show that the two fires were connected.
They were also connected to an incendiary device left at a different ballot drop box in Vancouver on October 8.
No ballots were damaged in that incident.