Three more people, including at least one volunteer firefighter, were arrested Saturday in connection with the largest residential arson in Maryland history, a collection of blazes that caused $10 million in damage to houses in an upscale development.

Authorities would not immediately discuss a possible motive in the Dec. 6 fires at the Hunters Brooke (search) development, where many homes were still under construction.

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. Authorities said the men have been charged with arson and are scheduled to appear in federal court Monday.

Charges have already been filed against Aaron Speed (search), 21, a security guard who told authorities he was upset by the way he had been treated by his employer.

The fires destroyed 10 houses and damaged 16 others. They broke out almost simultaneously over a 10-acre site, leading police to believe more than one person was involved.

One of the suspects arrested Saturday, Jeremy Parady, was listed on the Web site for the Accokeek Volunteer Fire Department (search) as a "riding member."

Wayne Jordan, president of the fire department, acknowledged one of his members was arrested in the case but declined to say if it was Parady.

Jordan said the arrested firefighter was a probationary member of the department and has been suspended. He declined to say whether the man was one of the firefighters who helped battle the blazes at the development. The department has about 50 active members.

The chief of the department, Jeff Cox, declined to comment Saturday.

Few details were known about the other suspects. A man who answered the phone at Walsh's home and identified himself as Walsh's father said only: "We're 100 percent sure that he is innocent."

A former neighbor of Everhart, Scott 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.

Speed, who worked at the development for Security Services of America, has told investigators he was upset his employer did not show enough sympathy after his infant son died this year, according to court documents.

Speed told investigators that his son, one of twins, died in April, when he was about three months old. His wife's stepfather said the baby, Christian, died of intestinal complications.

Speed came under suspicion after allegedly failing a polygraph and telling investigators he was home in bed when a colleague phoned to tell him of the fires, although phone records showed the call went through a cell phone tower closer to the scene, according to court documents.

Initially, there had been speculation the fires were set by environmental extremists because some environmental groups had complained the houses threatened a nearby bog. But no evidence has been found to support that theory, police said.

Security Services of America has said only that it is cooperating with authorities. The security firm is based in North Carolina and has operations in more than 20 states.