The man suspected of setting fires in ballot drop boxes in Oregon and Washington state is an experienced metalworker and may be planning additional attacks, authorities said Wednesday.

Investigators believe the man who set the incendiary devices at ballot boxes in Portland, Ore., and nearby Vancouver, Wash., had a "wealth of experience" in metal fabrication and welding, said Portland Police Bureau spokesperson Mike Benner.

The way the devices were constructed and how they were attached to the metal drop boxes showed that expertise, Benner said.

Authorities described the suspect as a white man, age 30 to 40, who is balding or has very short hair.

Police previously said surveillance video showed the man driving a black or dark-coloured 2001 to 2004 Volvo S-60. The vehicle did not have a front licence plate, but it did have a rear plate with unknown letters or numbers.

The incendiary devices were marked with the message "Free Gaza," according to a law enforcement official who spoke to The Associated Press on the condition of anonymity because they were not authorized to discuss an ongoing investigation.

A third device placed at a different drop box in Vancouver earlier this month also carried the words "Free Palestine" in addition to "Free Gaza," the official said.

Effort to identify suspect, motive for attacks

Investigators are trying to identify the person responsible and the motive for the suspected arson attacks, which destroyed or damaged hundreds of ballots at the drop box in Vancouver on Monday when the box's fire suppression system didn't work as intended.

Authorities are trying to figure out whether the suspect actually had pro-Palestinian views or used the message to try to create confusion, according to the official.

Surveillance images captured a 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 was extinguished quickly thanks to the box's suppression system and a nearby security guard, police said. Just three of the ballots inside were damaged.

The ballot box in Vancouver that burned also had a fire suppression system inside, but it failed to prevent hundreds of ballots from being scorched, said Greg Kimsey, the long-time elected auditor in Clark County, Wash.

Kimsey said Tuesday that the exact number of destroyed ballots wasn't known, and that about 475 damaged ballots had been retrieved from the box.

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. Kimsey urged voters who dropped their ballots in the transit centre box between 11 a.m. local time Saturday and early Monday to contact his office for a replacement ballot.

Authorities in Portland said Monday that enough material from the devices was recovered to show that the two fires were connected — and that they were connected to an Oct. 8 incendiary device at a different ballot drop box in Vancouver. No ballots were damaged in that incident.

Voters in Washington are encouraged to check the status of their ballots online to track their return status. If a returned ballot is not marked as "received," voters can print a replacement ballot or visit their local elections department for a replacement, the secretary of state's office said.