The three men were arrested early Saturday, federal and local officials said. They would not immediately discuss a possible motive.
The U.S. attorney's office in Baltimore identified the men as Patrick Stephen Walsh, 20, of Fort Washington; Jeremy Daniel Parady, 20, of Accokeek; and Michael McIntosh Everhart, 20, of Waldorf. In a recorded message, a spokeswoman for the office said the men had been charged with arson.
The three suspects were to appear Monday before a U.S. magistrate judge in federal court in Greenbelt, the Charles County sheriff's office said.
Wayne Jordan, president of the Accokeek Volunteer Fire Department, said the arrested firefighter was a probationary member of the department, which helped fight the Dec. 6 fires.
Jordan declined to name the firefighter, who has been suspended, and he wouldn't confirm whether the man responded to the fire in the upscale residential neighborhood that was under construction in Charles County.
Jeremy Parady was listed on the Web site for the fire department as a "riding member." The chief of the department, Jeff Cox, declined to comment Saturday. The department has about 50 active members.
Inside the one-story brick firehouse, Jordan, Cox and other members of the department huddled Saturday in the chief's office as they put together a statement. In a back room, four young volunteers lounged on couches, watching a movie on a big-screen TV. Three fire trucks and two ambulances were parked in the closed garage.
Few details were known about the other suspects Saturday.
"We're 100 percent sure that he is innocent," said a man who would only identify himself as Walsh's father when reached at his home in Fort Washington.
A former neighbor of Everhart's said the family had lived in a small Waldorf subdivision until about a year ago. Scott Ackerman said he believed the family moved to the same neighborhood where another man charged Thursday with arson, Aaron L. Speed, lived.
Ackerman said Everhart spent a lot of time working with friends on a car outside his house. "He's just a typical young kid, playing music and working on the car," he said.
On Friday, a federal magistrate ordered the U.S. Marshals Service to continue to detain Speed, 21, who worked for Security Services of America, until a hearing Tuesday. Speed has also been charged with arson.
The security guard said he was upset his employer did not show enough sympathy after his infant son died this year, according to court documents.
Speed was hired to protect the Hunters Brooke development in suburban Washington, where a string of fires destroyed 10 houses and damaged 16 others. No one was hurt in the fires; many of the houses were under construction.
Speed told authorities "that he was present at the location, along with others with whom he was acquainted, while the fires were being lit," according to an affidavit.
"Speed claimed that he knew of the plan by others known to him to set a fire at the location," the affidavit continued. "He also asserted that he told others how to gain access to the site."
Speed made the statements after failing a polygraph test Thursday. Earlier in the day, Speed told WUSA-TV in Washington that police suspected the wrong man: "Everything that I'm doing, I'm doing willingly to prove to them that I am innocent."
The scale of the arson — the fires broke out almost simultaneously over a 10-acre site — led investigators to believe that more than one person may have been responsible. Authorities believe the fires were set using an accelerant and a propane torch.
Police said there was no evidence to support an early theory that the fires were set by environmental extremists; some environmental groups had complained the houses threatened a nearby bog.
Speed, who grew up in Waldorf, told police he left his security job from August to October because of SSA's "indifference to the death of his infant son," according to the court papers.
When asked by investigators who might have started the fire, Speed said: "Someone who works at the site and recently experienced a great loss."
The company has said only that it's cooperating with authorities.
Speed told investigators that his son died in April, when he was about two and a half months old.