Officials were investigating a suspicious fire that gutted the prosecutor's office in this rural county Thursday, three days after an arsonist torched the historic courthouse just half a block away.

Police warned jumpy residents in this city of 8,300 people to be on the lookout for a dangerous suspect who may be someone they know.

Nobody was injured in either fire, the latest of which started shortly after 4 a.m. in the prosecutor's office in a one-story brick building that houses several other law offices in Lancaster's small downtown, authorities said.

Police Capt. Harlean Howard said witnesses to the fires have been interviewed but urged more to come forward. She also said experience shows the suspect will likely follow the investigation closely in media reports and in conversations.

"Until he is apprehended, he continues to pose a threat, not only to other potential buildings and property, but also to the property and safety of friends and family members as well. The reason for this is that under the right set of circumstances, it is possible he could strike out, unpredictably, against someone he knows," Howard said.

"He may make comments or offer reasons why he believes the fires were started such as, 'What do they expect? They didn't have any alarm system'; 'It was probably just some kids'; 'The court here is corrupt, biased, etc.'; 'No one was hurt,"' Howard said.

The 180-year-old Lancaster County Courthouse, designed by Washington Monument creator Robert Mills, suffered heavy damage in a blaze Monday that officials believe was intentionally set.

Randy Biggs, supervisor for the Bureau of Alcohol, Tobacco and Firearms, said Thursday's fire also was arson. Howard would only say the latest fire was considered suspicious.

"We've got a wonderful town and a wonderful county, but we've got sick people out there," said Mayor Joe Shaw.

He said a murder trial was expected to begin next week at the courthouse but did not have details on that and said officials have not linked the fires to anyone involved in that case. County officials, meanwhile, had proposed a seven-year sales tax hike to pay for a new, $33 million downtown courthouse that voters are to consider in November, Shaw said. There had been plans to turn the old courthouse into a museum.

Lancaster is near the North Carolina state line, about 40 miles south of Charlotte, N.C., and 60 miles north of Columbia. It's the seat of a rural county that is home to less than 65,000 people and which is beset by unemployment that stood at 10 percent in June.

Johnny Parker, who owns a car repair and detailing shop about a block from the courthouse, said the two fires have residents worried.

"We're not like this," said Parker, 48, gesturing toward recent landscaping and remodeling along the city's main street. "We're an old historic town. ... A lot of nice people live in Lancaster."