Sources say the devices used in the ballot box fires were marked “Free Gaza.”

Ballot boxes in Oregon and Washington were set on fire with incendiary devices early Monday morning in what authorities believe is a related incident, police said.

The devices used in the arson attacks were marked with “Free Gaza Strip,” two sources familiar with the investigation told ABC News.

The ballot box fires, which occurred near the Oregon-Washington border, are also linked to a third ballot box incident that occurred earlier this month in Vancouver, Washington, police said.

Sources said the incendiary device used in the first arson on October 8 bore the words “Free Gaza” and “Free Palestine.” Two subsequent devices detonated early Monday morning in Vancouver, Washington, and nearby Portland, Oregon, carried the slogan “Free Gaza.”

One source told ABC News that it was unclear whether the labels reflected the views of a pro-Palestinian activist or were someone trying to manipulate existing divisions in the US.

In the first reported incident Monday, Portland police responded to a ballot box arson around 3:30 a.m. local time, police said. Multnomah County Elections Security responded and extinguished the fire, officials said.

“Our officers quickly determined that there was an incendiary device attached to the ballot box and that was what started the fire,” Portland Police Bureau spokesman Mike Banner said at a briefing Monday.

The bureau’s explosive ordnance disposal unit disarmed the device, police said.

Fire suppression equipment prevented further damage and protected “virtually all of the ballots,” although three were damaged, the Multnomah County Elections Department said in a news release.

Election officials said they are contacting the three affected voters so they can receive new ballots.

Multnomah County Elections Director Tim Scott told ABC News that the 409 ballots inside the ballot box “were not damaged” and were preserved by fire suppression equipment.

Oregon Secretary of State LaVonne Griffin-Valade said the limited impact on ballots “shows that our systems are safe and secure.”

“Make no mistake, an attack on the ballot box is an attack on our democracy and is completely unacceptable,” Griffin-Valade said in a statement. “Whatever the motivation for this incident, there is no justification for any attempt to disenfranchise voters.”

No other ballot drop boxes or official ballot drop-off locations in Multnomah County were affected, the department said.

The investigation is being conducted by the Portland Fire Investigation Unit.

About half an hour later, around 4 a.m. local time, police in Vancouver, Washington, responded to a report of a smoking and burning ballot box, police said.

“Officers arrived to find a suspicious device next to a burning box,” the Vancouver Police Department said in a statement.

The fire was extinguished and explosive disposal personnel recovered the device safely, police said.

“Hundreds” of ballots are believed to have been damaged Monday, although the official figure has not yet been determined, Clark County Auditor Greg Kimsey said at a press briefing.

Fire suppression devices are also installed at ballot drop boxes in the county, although they do not appear to be working properly, Kimsey said, adding that they are going to try to purchase more advanced fire suppression devices.

The Clark County Auditor’s Office will work to ensure affected voters receive replacement ballots in a timely manner, officials said.

“We take the safety of our election workers seriously and will not tolerate threats or acts of violence aimed at undermining the democratic process,” Washington Secretary of State Steve Hobbs said in a statement. “I strongly condemn any terrorist attacks aimed at disrupting legitimate and fair elections in Washington State. Despite this incident, I have full confidence in our county elections official’s ability to ensure a safe election in Washington for all voters.”

Monday’s incident is similar to an Oct. 8 incident in Vancouver in which a ballot box was smoking and burning with a “device” next to it, interim Vancouver police Chief Troy Price said during a press briefing.

“We believe the incident here (in Portland) is related to the two incidents in Vancouver,” Benner said.

Police have identified a suspicious vehicle, possibly a Volvo, that was captured on surveillance footage near a ballot box in Portland, Benner said.

“We know that these types of actions are targeted and intentional, and we are concerned that these intentional actions are attempting to influence the election process,” she said at a press briefing Monday. “We are committed to stopping this behavior and are working to do so today.”

The FBI is also investigating the incidents “to determine who is responsible,” an agency spokesman said.

Portland Gov. Tina Kotek praised the “quick response” of law enforcement and election officials.

“Voter intimidation or any criminal activity intended to undermine the upcoming election is un-American.” And this is unacceptable,” she said in a statement on Channel X.

Both Oregon and Washington are among several states that vote by mail, with ballots returned by mail or in an official drop box. There are also voting centers in Washington DC open to accept ballots.