Updated at 6:05 p.m.

A ballot drop box outside the Boston Public Library was lit on fire early Sunday morning, officials say, in what Secretary of State William Galvin called a “deliberate attack.”

Galvin said the FBI is investigating the incident, which took place at around 4 a.m. at the polling location in Copley Square.

“I'm disturbed by it. It's a crime,” Galvin said in an interview with GBH News Sunday. “We're very concerned about the security of drop boxes.”

The Boston Election Department said the drop box contained 122 ballots, 87 of which were able to be recovered and processed. Galvin's office said voters who used the Copley Square ballot box between the hours of 2:30 p.m. on Saturday and 4 a.m. on Sunday should track their ballots online at www.trackmyballotMA.com. Anyone who cannot confirm the receipt of their ballot should call the Election Department immediately, Galvin’s office said.

Galvin said the ballot box was damaged by an “incendiary device,” and the FBI is looking into what was used. “Obviously it was something strong enough to sustain a fire in a closed box,” Galvin said. “It did do significant damage to the interior lining of the box, so it had to be a fairly high amount of heat.”

Affected voters will be mailed a replacement ballot by the City of Boston and will have the option of casting that replacement ballot or voting in person until 8 p.m. on Election Day, according to Galvin’s office.

“If it has been accepted, then they're OK, and they will be given the opportunity to redo if necessary,” Galvin said. “And obviously we're reaching out to the people with damaged ballots that we can determine.”

Galvin has also directed local election officials across the state to increase security around ballot drop boxes, including emptying drop boxes frequently and employing drop box guards and video surveillance.

“I issued an emergency directive suggesting that local election officials have to be extra vigilant when it comes to the location of drop boxes, and if necessary, to take them in at night,” Galvin told GBH News. “And have security available to make sure they're protected.”

In a joint statement with Mayor Marty Walsh, Galvin asked voters not to be intimidated by the act.

“What happened in the early hours of this morning to the ballot drop box in Copley Square is a disgrace to democracy, a disrespect to the voters fulfilling their civic duty, and a crime,” the statement reads. “Our first and foremost priority is maintaining the integrity of our elections process and ensuring transparency and trust with our voters, and any effort to undermine or tamper with that process must be prosecuted to the fullest extent of the law.”