On Sunday, the Hillsborough police said the fire had been caused by a firebomb thrown through a window of the office, which is in a shopping center about 14 miles outside Durham.

The damage was not noticed until Sunday, when a business owner called the authorities around 9 a.m., the police said. Gov. Pat McCrory said in a statement that no one had been injured.

“This highly disturbing act goes far beyond vandalizing property; it willfully threatens our community’s safety via fire, and its hateful message undermines decency, respect and integrity in civic participation,” Mayor Tom Stevens of Hillsborough said in a statement. “Our law enforcement officials are responding quickly and thoroughly to investigate this reprehensible act and prosecute the perpetrators.”

The Hillsborough police said they were investigating the attack along with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.