BALTIMORE – A security guard who worked at a development where a series of fires burned several homes has been arrested and charged in the case, a source close to the investigation said Thursday.

Aaron Speed, 21, was to appear Friday in federal court, the source said. The exact charges and alleged motive were not immediately known.

Speed is an employee of Security Services of America, a company hired to guard the upscale development outside the nation’s capital. Fires broke out there early Dec. 6, doing $10 million in damage.

No one was hurt, but a total of 26 houses were damaged, 10 of them extensively, in what authorities described as the largest residential arson case in Maryland history. Investigators said there was evidence that whoever was behind the arson tried to set 10 more blazes at the subdivision in the town of Indian Head, 35 miles south of Washington.

Early speculation was that the fires were set by environmentalists who believed the houses were a threat to a nearby bog. But no evidence has been found to support that theory, police said.

Because of the scale of the crime scene, authorities believe at least two people are responsible.

Authorities searched the home of Speed’s parents on Wednesday night and towed a car away, said David Jaillet, whose stepdaughter is married to the security guard. No one answered the door Thursday night at the house. A homemade “No Trespassing” sign was taped to the storm door of the ranch-style home.