Authorities, including the FBI, were investigating Monday after early morning fires were set in U.S. ballot drop boxes in Portland, Ore., and in nearby Vancouver, Wash., where hundreds of ballots were destroyed.

The Portland Police Bureau reported that officers and firefighters responded to a fire in one ballot drop box at about 3:30 a.m. PT and determined an incendiary device had been placed inside. Multnomah County elections director Tim Scott said a fire suppressant inside the drop box protected nearly all the ballots; only three were damaged, and his office planned to contact those voters to help them obtain replacements.

A few hours later, across the Columbia River in Vancouver, television crews captured footage of smoke pouring out of a ballot box at a transit centre. Vancouver is the biggest city in Washington's third congressional district, the site of what is expected to be one of the closest U.S. House races in the country, between first-term Democratic Rep. Marie Gluesenkamp Perez and Republican challenger Joe Kent.

"I hope the perpetrator of this reprehensible act is quickly apprehended — and local and federal law enforcement have my full support in working to keep our democratic process safe and secure," Gluesenkamp Perez said in a statement.

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She said she's requesting an overnight law enforcement presence posted at all ballot drop boxes in Clark County through election day.

"Southwest Washington cannot risk a single vote being lost to arson and political violence," she said in the statement.

Representatives for Kent's campaign didn't immediately return The Associated Press's phone and email messages seeking comment.

Fires 'direct attack on democracy': county auditor

Clark County auditor Greg Kimsey in Vancouver, Wash., told the AP that the ballot drop box at the Fisher's Landing Transit Center also had a fire suppression system inside, but for some reason it wasn't effective. Responders pulled a burning pile of ballots from inside the box, and Kimsey said hundreds were lost.

"Heartbreaking," Kimsey said of the incident. "It's a direct attack on democracy."

There were surveillance cameras that covered the drop box and surrounding area, he said.

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The last ballot pickup at the transit centre drop box was at 11 a.m. PT Saturday, Kimsey said. Anyone who dropped their ballot there after that was urged to contact the auditor's office to obtain a new one.

The office will be increasing how frequently it collects ballots, Kimsey said, and changing collection times to the evening, to keep the ballot boxes from remaining full overnight, when similar crimes are considered more likely to occur.

An incendiary device was also found on or near a drop box in downtown Vancouver early on Oct. 8. It did not damage the box or destroy any ballots, police said.

FBI asking public for information

In a statement, the FBI said it is co-ordinating with U.S. federal, state and local partners to investigate the two incidents. Anyone with information is asked to contact the nearest FBI office, provide information through tips.fbi.gov or call 1-800-CALL-FBI (800-225-5324).

Washington Secretary of State Steve Hobbs said the state would not tolerate threats or acts of violence meant to derail voting.

"I strongly denounce any acts of terror that aim to disrupt lawful and fair elections in Washington state," he said.

Voters were encouraged to check their ballot status online at www.votewa.gov to track its return status. If a returned ballot is not marked as "received," voters can print a replacement ballot or visit their local elections department for a replacement, the secretary of state's office said.

Washington and Oregon are both vote-by-mail states. Registered voters receive their ballots in the mail a few weeks before elections and then return them by mail or by placing them in ballot drop boxes.

In Phoenix last week, officials said roughly five ballots were destroyed and others damaged when a fire was set in a drop box at a U.S. Postal Service station there.