Authorities in Portland, Oregon, and Vancouver, Washington, said they were investigating after two ballot drop boxes burst into flames early Monday morning.

In Portland, an incendiary device was placed in the ballot box and 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, just over the state line and about a 20-minute drive from Portland.

According to the police bureau, officers in Portland responded to reports of a burning ballot box around 3:30 a.m. Pacific Time. The mailbox where voters can drop their vote upcoming electionwas located in southeast Portland near the Multnomah County Elections Division office, according to CBS affiliate KOIN-TV.

When officers arrived, the fire had already been extinguished by security forces working in the area. Officials discovered an incendiary device in the ballot box that they believed was used to start the fire. According to the police, the explosives squad was called to the scene of the accident and the device was cleared.

The Portland Fire Investigations Unit is working with police to investigate the incident.

“The collaboration leverages the specialized training and skills required to investigate fires, including analytical equipment and dogs to detect accelerant,” the police statement said.

Authorities have asked anyone with information about the Portland ballot box fire to contact Detective Meredith Hopper at the police office or report tips through the special fire investigation unit line.

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

ABC affiliate KATU-TV released several images showing thick smoke billowing from the ballot box and burning ballots being thrown 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 working 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 28, to determine who is responsible,” the agency said in a statement to CBS News.

Clark County Auditor Greg Kimsey told The Associated Press that the ballot box in Fisher’s Landing had a fire suppression system, but for some reason it was not effective. Kimsey said hundreds of ballots were destroyed in the fire, the AP reported.

It was at least the second time in October that Vancouver authorities had dealt with an incident like this. Earlier this month, the city’s Metro Explosive Disposal Unit destroyed another suspicious device found near a ballot box early one morning, KOIN-TV reported at the time. According to the station, the ballot box was not damaged in this case and no one was injured, police said.