Ballot boxes in Oregon and Washington were set on fire early Monday, police in both states said.
It is unclear if the two arson incidents, which occurred near the Oregon-Washington border, are connected.
In the first reported incident, a ballot box in Portland, Oregon, was set on fire with an incendiary device, police said.
Portland police responded to reports of a fire at a ballot box around 3:30 a.m. local time on Monday. Security at the Multnomah County Elections Division responded and extinguished the fire, officials said.
"Officers determined an incendiary device was placed inside the ballot box and used to ignite the fire," the Portland Police Bureau said in a statement.
The bureau's explosive disposal unit cleared the device, police said.
Fire suppressant prevented further damage and protected "virtually all the ballots," though three were damaged, the Multnomah County Elections Division said in a press release.
Elections officials will contact the three impacted voters so they can receive replacement ballots, the division said.
"We have multiple systems and security measures in place to ensure your ballot is safe,'' Multnomah County Elections Director Tim Scott said in a statement.
Oregon Secretary of State LaVonne Griffin-Valade said the limited impact to ballots "shows that our systems are safe and secure."
"Make no mistake, an attack on a ballot box is an attack on our democracy and completely unacceptable," Griffin-Valade said in a statement. "Whatever the motivation behind this incident, there is no justification for any attempt to disenfranchise voters."
No other ballot boxes or official drop sites in Multnomah County were affected, the division said.
The Portland Fire Investigations Unit is investigating.
About a half hour later, around 4 a.m. local time, officers in Vancouver, Washington, responded to a report of a ballot box that was smoking and on fire, police said.
"Officers arrived and located a suspicious device next to the box," which was on fire, the Vancouver Police Department said in a statement.
The fire was extinguished, and members of the Metro Explosive Disposal Unit safely collected the device, police said.
A suspected incendiary device was placed in the drop box and that "some ballots" are believed to have been damaged, according to the Washington Secretary of State's Office. The Clark County Auditor's Office will be working to ensure impacted voters have replacement ballots in time, it said.
"We take the safety of our election workers seriously and will not tolerate threats or acts of violence that seek to undermine the democratic process," Washington Secretary of State Steve Hobbs said in a statement. "I strongly denounce any acts of terror that aim to disrupt lawful and fair elections in Washington state. Despite this incident, I have complete confidence in our county elections official's ability to keep Washington's elections safe and secure for all voters."
The FBI is investigating both incidents "to determine who is responsible," an agency spokesperson said.
Both Oregon and Washington are one of several vote-by-mail states, with ballots returned by mail or at an official drop box. Washington also has voting centers open to accept ballots.