After a local Republican Party headquarters in Orange County, N.C., was vandalized and set alight, Republican presidential nominee Donald Trump tweeted that Hillary Clinton's supporters were responsible. Clinton called the attack "horrific and unacceptable." (Reuters)

A firebomb burned the inside of a local Republican Party headquarters in Orange County, N.C., on Saturday night, and someone spray-painted a swastika and a warning for GOP members to leave town, police and party officials said Sunday.

The town of Hillsborough, N.C., where the headquarters is located, said in a statement that police were investigating the incident, along with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives. No damage estimates were available Sunday, and authorities did not identify any suspects.

A blaze ignited when someone threw a bottle filled with flammable liquid through the office’s front window, burning furniture, blackening the walls and melting campaign signs before going out on its own, authorities said. On the side of the building next door, someone scrawled a swastika and wrote “Nazi Republicans leave town or else” in black spray-paint.

The state party tweeted out pictures of the damage on Sunday:

With just three weeks left until Election Day, the attack adds yet another layer of tension to a campaign season that has been marked by bitter fighting and extreme anger on both sides of the aisle. In the days since a video emerged showing Donald Trump bragging about sexually assaulting women, the Republican nominee has ratcheted up claims that the election is “rigged” against him and called for Hillary Clinton to be jailed over her use of a private email server.

Trump went on the offensive again Sunday, alleging in a tweet that “animals” representing the Democratic nominee attacked the Republican headquarters in Hillsborough because Trump was winning.

Clinton tweeted her condolences, calling the attack “horrific and unacceptable.”

The North Carolina Republican Party thanked both candidates for their support. In the county where the headquarters is located, registered Democrats outnumber registered Republicans 3 to 1, the Associated Press reported.

According to the town of Hillsborough, the attack took place sometime between 9 p.m. Saturday night and Sunday morning. Another business owner discovered the heavy smoke and fire damage and alerted police around 9 a.m. Sunday, the town said in a statement. The headquarters is located in a shopping center known as the Shops at Daniel Boone, and sits next to an optometry office and a store that sells party decorations.

“This highly disturbing act goes far beyond vandalizing property,” Hillsborough Mayor Tom Stevens (D) said in a statement. “It willfully threatens our community’s safety via fire, and its hateful message undermines decency, respect and integrity in civic participation.”

Daniel Ashley, chairman of the Orange County Republican Party, said in a video posted Sunday by the state GOP that someone threw a molotov cocktail through the window and the resulting fire melted a printer and other materials. Nearly all the office’s campaign signs for Trump and other local Republicans candidates were destroyed, as were sample ballots for early voting, which starts Thursday in the state.

Ashley said the party kept the office open year-round in an attempt to make inroads into the heavily Democratic county.

“We will not be intimidated,” he said. “We’ve let some people know we’re in town — because they want to run us out of town, and we’re not going anywhere.”

Hillsborough, N.C., police are investigating "arson and graffiti" that destroyed the local GOP headquarters in Orange County overnight on Saturday, Oct. 15. The local chairman streamed a video amid the damage on Oct. 16, which includes charred campaign signs, melted office supplies and graffiti calling them "Nazi Republicans." (Facebook/North Carolina GOP via Storyful)

Dallas Woodhouse, the state Republican Party’s executive director, called the attack “political terrorism,” and said a molotov cocktail smashed through a window that had displayed a sign reading “Freedom spoken here.”

“The office itself is a total loss,” Woodhouse told the Charlotte Observer. “The only thing important to us is that nobody was killed, and they very well could have been.”

“This is a horrific, horrific act of political terrorism,” he said, “one that we will not succumb to and one that we will answer.”

The North Carolina Democratic Party condemned the attack, saying violence had no place in the political system.

Many people rallied to support the state GOP. On Sunday, David Weinberger, a senior researcher at Harvard’s Berkman Klein Center for Internet & Society, launched a GoFundMe campaign on behalf of Democrats to help the headquarters reopen as soon as possible. More than 500 donations poured in and the campaign surpassed its fundraising goal of $10,000 in about 40 minutes, Weinberger said.

“This is not how Americans resolve their differences,” Weinberger said on the GoFundMe page. “We do not resort to violence by individuals or by mobs.”

The campaign drew mixed reactions on Twitter.

Woodhouse, the state party director, recommended all GOP offices in North Carolina close by sundown on Sunday and urged them to take extra caution going forward. Later in the evening Sunday, North Carolina Gov. Pat McCrory (R) had staffers deliver new campaign materials to Orange County Republicans.

In a separate incident, at least 20 vehicles were vandalized with spray paint outside a Trump rally in Bangor, Maine, on Saturday afternoon, the Bangor Daily News reported.

Violence has cropped up at other campaign events this year, with numerous fights breaking out at Trump rallies around the country. In June, protesters pelted Trump supporters with eggs and water balloons before punching them outside one of the candidate’s events in San Jose And last month, a man leaving a Trump rally in Asheville, N.C., struck a 69-year-old anti-Trump demonstrator who was wearing an oxygen tank to treat a lung disease.