Ballot drop boxes set on fire in Portland, Ore., and Vancouver, Wash.

Ballot drop boxes in Portland, Oregon, and neighboring Vancouver, Washington, were set on fire early Monday morning, police said, just over a week out from Election Day.

Local officials said three ballots were damaged in Portland while potentially “hundreds” were affected in Vancouver.

The Portland Police Bureau said an "incendiary device" was placed inside of a sidewalk ballot box in the Central City district. By the time officers responded to reports at about 3:30 a.m. local time, security personnel in the area had extinguished the flames.

A ballot box on fire in Vancouver, Wash.KGW

"Officers determined an incendiary device was placed inside the ballot box and used to ignite the fire," a statement from the Portland police read, adding that the bureau's Explosive Disposal Unit removed the mechanism.

Portland police were continuing to investigate the incident.

A statement from the Multnomah County election office said that "fire suppressant inside the ballot box protected virtually all the ballots."

"Voters should be assured that even if their ballots were in the affected box, their votes will be counted," the statement read.

At around 4 a.m. local time, about a half-hour after the reported fire in Portland, police in Vancouver also responded to a ballot box fire.

Flames and smoke could be seen emanating from the box near the Fisher's Landing Transit Depot, reported NBC News affiliate KGW in Portland.

Vancouver police said officers arrived to smoke and flames and located a "suspicious device" next to the box. The fire was put out, and the city's Metro Explosive Disposal Unit collected the device.

The FBI was investigating, and no further details were immediately available, Vancouver police said.

Clark County auditor Greg Kimsey said that potentially "hundreds" of ballots could be damaged. He added that anyone who placed a ballot into the box after 11 a.m. Saturday should contact the office to confirm the status of their ballot.

It's unclear if the two incidents were related and authorities have not ascribed a motive, but they come as election officials across the country have said they are on high alert. For many localities, once-routine election business has become a security challenge, with some adding law enforcement and even drones and bulletproof vests out of concerns that extremists inspired by conspiracy theories could seek to disrupt voting or vote counting.

Earlier this month, the Vancouver bomb squad responded to reports of a "suspicious device" near a downtown ballot box, police said. The device was "safely removed" and the ballot box itself was "not compromised," according to a news release.

Vancouver is in Washington's 3rd Congressional District, where Democratic Rep. Marie Gluesenkamp Perez is facing off against Donald Trump-backed Republican challenger Joe Kent in a highly competitive House race.

Several ballots were also damaged in Phoenix last week after a U.S. Postal Service mailbox was lit on fire. Police arrested a suspect, who admitted committing arson but said the incident was not politically motivated.