The FBI are investigating a fire that was started on Sunday in a Boston ballot drop box holding more than 120 voting papers.
Massachusetts election officials said it appeared to have been a “deliberate attack”.
The fire was started about 4am in a ballot drop box outside the Boston public library, said the office of Massachusetts secretary of the commonwealth, William Galvin.
There were 122 ballots inside the box when it was emptied on Sunday morning, and 87 of them were still legible and able to be processed, the office said.
The box had last been emptied about 2.30pm on Saturday.
In a joint statement, Mr Galvin and Boston Mayor Marty Walsh called it a “disgrace to democracy, a disrespect to the voters fulfilling their civic duty, and a crime".
"Our first and foremost priority is maintaining the integrity of our elections process and ensuring transparency and trust with our voters," they said.
"And any effort to undermine or tamper with that process must be prosecuted to the fullest extent of the law.
"We ask voters not to be intimidated by this bad act, and remain committed to making their voices heard in this and every election.”
Boston Police said an arson investigation was under way and released surveillance images of a person near the ballot box at the time, urging the public to help identify them.
Officers called to the scene saw smoke coming out of the box before firefighters extinguished the blaze.
"It is a top priority of our offices to help maintain the integrity of the election process in Massachusetts by aggressively enforcing federal election laws," the FBI said.
Voters can go online to see whether their ballot was processed.
Those who used the box between Saturday afternoon and 4am on Sunday, who cannot confirm the status of their ballot online, should contact the Boston Elections Department immediately, officials said.
Voters whose ballots were affected can either vote in person or by a replacement ballot that will be mailed to them, officials said.
If those voters do not submit a new ballot, “their original ballot will be hand-counted to the extent possible", Mr Galvin's office said.
