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Police say two incidents in which ballot drop boxes were set on fire early Monday morning in Portland, Oregon, and nearby Vancouver, Washington, appear to be related.

An identical black Volvo sedan was spotted at both crime scenes, and an “incendiary device” used in Portland was “of a similar nature” to what happened in Vancouver, police said. They added that the circumstances were consistent with an attempt on Oct. 8 to set fire to another ballot box in Vancouver.

“We do not know the motive behind these actions,” Portland Police Bureau Deputy Chief Amanda McMillan said in a statement Monday. “We know that such actions are targeted and deliberate, and we are concerned that this deliberate action seeks to influence the electoral process. We are committed to stopping this type of behavior.”

The fires that broke out in the early morning hours at a ballot box at a transit center in Vancouver and outside Multnomah County election offices in Portland damaged three ballots in Portland and may have endangered hundreds in Vancouver.

Local officials said the incident was unprecedented.

“I have been involved in elections in Multnomah County for 16 years. I’ve been voting in elections for 22 years,” Tim Scott, the county’s elections director, told reporters about the Portland incident. “As far as I know, this is the first incident.”

Federal officials are helping local police investigate the ballot fires.

“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, and determine who is responsible for it,” the authority said The Independent.

The Multnomah County Sheriff’s Office said it will increase patrols around mailboxes in the county.

Washington officials urged the public to monitor the status of their votes on VoteWA.gov and sought to reassure the public of the integrity of the election.

“Washington Democrats trust law enforcement and the Clark County auditor to find those responsible and hold them accountable,” Washington Democratic Chairman Shasti Conrad said in a statement Monday. “Voting is safe and easy, and Washington State is proud to have one of the most secure voting systems in the country. We take the rights of every voter very seriously.”

Intelligence officials began warning this summer that “perceptions of voter fraud” could trigger “mobilization toward violence,” including attacks on voting infrastructure.

The Department of Homeland Security said it was monitoring a group online seeking information about “incendiary and explosive devices” to make processing ballots “impossible.” WIRED reported.