A fire inside an official Los Angeles County ballot drop box is being investigated as arson and has been reported to the FBI, authorities said Monday.

The fire was reported Sunday night in a curbside box outside a library in suburban Baldwin Park, east of Los Angeles.

Firefighters put out the fire, cut open the box and pulled out envelopes, some charred.

The county registrar-recorder’s office said the items were being reviewed “to determine the appropriate notifications to voters whose ballots may have been impacted and will ensure they can exercise their fundamental right to vote.”

Ballots were last collected from the box shortly after 10 a.m. Saturday. Voters who dropped off ballots after that time were asked to call the registrar recorder.

County Supervisor Hilda L. Solis said the fire had “all the signs of an attempt to disenfranchise voters and call into question the security of our elections.”

The frequency of ballot pickups at all boxes was being increased immediately.

Local voter John Rios told KABC-TV the incident made him angry.

“I’m 80 years old. I’ve been voting since I was 19. I’ve never seen something like this,” he said.

Janice Hahn, also a member of the county Board of Supervisors, said the fire was reprehensible.

“Burning ballots is not just vandalism, it is an attack on our democracy and on our residents’ right to vote,” she said. “Whoever did this must be found and prosecuted to the fullest extent of the law.”

Baldwin Park Mayor Manuel Lozano noted frustrations in America about the voting process and told KABC the incident sends “a very bad message.”

The fire came at the end of a week of controversy over unofficial ballot drop boxes placed in various locations in California by the state Republican Party.

California’s secretary of state said Friday that the GOP removed the unstaffed, unofficial ballot drop boxes, and subpoenas were being issued to get more information about them as the state attorney general looks into possible election law violations.

Republicans confirmed they pulled boxes improperly labeled “official.” But they say they will continue to use dozens of other boxes without the labels that have been sent to various counties.