Ballot boxes in Oregon and Washington may have been set on fire by an anti-Israel protester, as investigators found a note that said “ALL DROP BOXES WILL BURN. FREE GAZA,” The Post has learned.

Between 3 and 4 a.m. on Monday, fires caused by incendiary devices were discovered in ballot boxes in Vancouver, Wash. and neighboring Portland, Oregon.

A built-in fire suppressant stopped most of the flames at the Portland location, but hundreds of ballots were destroyed at the drop box at Fisher’s Landing Transit Center in Vancouver.

The FBI is now heading the investigation into the two Monday fires, which are believed to be linked and also connected to a third attempted arson attack earlier in the month in Vancouver.

Election officials and law enforcement have remained largely mum about their investigation.

They’ve directed those who used the ballot boxes over the weekend to track their ballots online and to contact the local auditor’s office for replacement ballots if needed.

But according to an election official with knowledge but not authorized to speak on record, a political letter was discovered in a ballot box at the Vancouver Mall Parking lot location just a few hours after the fires were put out.

According to the source, a white piece of paper folded in half had been deposited in the box.

It is not confirmed if the message came from the suspect involved in the arson fires, but the New York Times reports that all three incendiary devices themselves had the message “Free Gaza” written on them.

The shopping center drop box is less than 15 minutes by car from the Fisher’s Landing Transit Center, where one of the fires occurred.

“Um, I’m not gonna have a comment on that at this time,” Clark County Auditor Greg Kimsey said when asked about the political letter.

The election source tells The Post that county and investigators have not publicly shared information about the investigation over fears it could keep voters from using the ballot boxes in the remaining days leading up to the election, or inspiring copycat attacks.

All elections in Washington and Oregon are vote-by-mail.

The arson attacks come amid a closely-watched congressional race rematch in Washington’s 3rd Congressional District between incumbent Rep. Marie Gluesenkamp Perez, a Democrat, and Joe Kent, a Republican US Army veteran. The two are polling neck-and-neck in the district.

By Monday evening, the Clark County Auditor announced that election office employees will monitor 24/7 all 22 drop box locations.

Vancouver Police announced additional patrols.

The Portland Police released photos of a suspect vehicle next to the torched Portland ballot box showing a black or dark-colored 2001-04 Volvo S-60.

The back license plate is not legible in the CCTV footage.