Authorities in Portland, Oregon, and Vancouver, Washington, said they've opened investigations after two ballot boxes went up in flames early Monday morning.

In Portland, an incendiary device was placed inside the ballot box and used to set it on fire, the Portland Police Bureau said in a statement. Police said a suspicious device was found next to the ballot box in Vancouver, which is just over the state line and about 20 minutes from Portland by car.

Most ballots in the Portland box were recovered intact, but officials feared hundreds were lost in the Vancouver incident.

Officers in Portland responded to reports of a ballot box on fire at around 3:30 a.m. Pacific Time, according to police. The mailbox, where voters can cast their ballots in the upcoming election, was located in southeastern Portland near the Multnomah County Elections Division office, CBS affiliate KOIN-TV reported.

When officers arrived, the fire had already been extinguished by security guards who work in the area. Officers discovered an incendiary device inside the ballot box, which they determined had been used to ignite the blaze. The bomb squad was called to the scene and cleared the device, according to police.

A Multnomah County elections official on Monday confirmed to CBS News that most ballots in the box were undamaged and were being processed Monday.

"I am confirming that 409 ballots were in the box and undamaged and are being processed today. Three ballots were damaged and all three voters were contacted and provided a replacement ballot," according to Tim Scott, director of Multnomah County Elections.

The Portland Fire Investigations Unit is partnering with the police bureau to investigate the incident. "The collaboration leverages the specialized training and skills needed to investigate fires, including analysis equipment and accelerant detecting dogs," the police statement read.

The Portland Police Bureau released photos of a vehicle believed to be involved in the fire at the ballot box. The vehicle was described as a black or dark-colored 2001-2004 Volvo S-60. Investigators said they also believe the vehicle is tied to the incidents in the Vancouver, Washington, area.

Authorities have asked anyone with information about the ballot box fire in Portland to contact Detective Meredith Hopper at the police bureau, or report tips through the fire investigation unit's dedicated line.

The ballot box in Vancouver caught fire soon after the one in Portland. Vancouver police said they responded around 4 a.m. to reports of a ballot box on fire in the area of the Fisher's Landing Transit Center.

ABC affiliate KATU-TV published several images that showed thick smoke rising from the ballot box, in addition to burning ballots emptied out onto the sidewalk.

Vancouver police said they extinguished the fire and members of the city's Metro Explosive Disposal Unit "safely collected the device." The FBI is investigating.

"The FBI is coordinating with federal, state and local partners to actively investigate the two incidents in Vancouver, Washington, and Portland, Oregon, in the early morning hours of Monday, October 28th to determine who is responsible," the agency said in a statement to CBS News.

Clark County Auditor Greg Kimsey told The Associated Press the ballot box at Fisher's Landing had a fire suppression system inside of it, but for some reason the system was not effective. Kimsey told CBS News that hundreds of ballots were impacted, but said officials didn't know the exact number yet.

Officials in Clark County said patrols around ballot boxes will be increased and ballot box observers will now watch the county's 23 ballot drop boxes around the clock until Election Day. The county will also assign election officials to pick up ballots at drop boxes more frequently, including every evening.

It was at least the second time in October that Vancouver authorities addressed an incident like this.

Earlier in the month, the city's Metro Explosive Disposal Unit destroyed a suspicious device found near a ballot box at around 4 a.m. on Oct. 8, Vancouver police said at the time. In that instance, officers responded to a "report of a possible arson" linked to smoke seen near a ballot box located in the vicinity of several federal buildings. The ballot box was not compromised and no one was injured, according to police.

contributed to this report.