Prince George's County fire investigators charged another volunteer firefighter with arson yesterday, bringing to 11 the number of volunteers accused of setting blazes in vacant houses, allegedly for the fun of extinguishing them.

Maj. Daniel A. Jarboe pronounced the investigation concluded yesterday with the arrest of Kenneth Armstrong, 22, a volunteer from the Boulevard Heights Company. Six volunteers from the District Heights Fire Company and four volunteers from Boulevard Heights were arrested this week and charged with setting fires in nine vacant houses near their respective fire stations since June 1989.

Armstrong, who officials said was implicated by one of the other volunteers, is charged with setting a 10th fire in a vacant house in the 4700 block of Huron Avenue in Boulevard Heights.

"This has been a continuous process with one person leading us to another," Jarboe said. "But I think we can say now that we've reached the end."

Fire investigators characterized some of the fires as "spur-of-the-moment" activities and said most of the accused volunteers had given fire investigators statements acknowledging their roles in the fires.

"There is some speculation that they wanted to be more a part of things and fight the fires," Jarboe said. "Who can really say why, whether it was boredom or what."

Investigators learned of the volunteers' alleged involvement after District Heights administrators discovered a diary on a table in the fire station that detailed the times and locations of several fires and named some of the people involved in setting them.

Fire officials denounced the crimes yesterday at a hastily called news conference and sought to minimize the damage to the reputation of the department.

The agency has worked for several years to integrate the volunteer firefighters, long a fixture in Prince George's County, with paid firefighters. Criminal background checks and more than 140 hours of training are required for volunteers, officials said.

"This is not normally the sort of thing that volunteers just go out and do," said Lt. Col. John E. Proels. "We have hundreds of dedicated volunteers and it would not be fair for this to tarnish their reputation."

Also charged with arson are Rodney Jerome Hall, 20, Robert Wayne Vasquez, 18, Demetrius Terry, 18, and Napoleon L. Queen, 21. Six of the volunteers arrested are 17-year-olds.

Hall and two juvenile firefighters charged with arson were suspended from the department this summer after they were accused of breaking into an apartment and assaulting a Prince George's County man during the statewide volunteer firefighters convention June 20 in Ocean City, Md., fire officials said.

The two juveniles are under investigation in that case but have not been charged, sources said. Hall is awaiting trial.

The fires that Hall and the two juveniles are accused of setting occurred before the incident in Ocean City, authorities said.

Jarboe said that investigators do not believe that the fires, which occurred largely in vacant and uninhabitable buildings, were set for money. No one was injured in the fires.

Investigators said they were still trying to put a dollar value on the property destroyed and noted that many of the properties had been slated for demolition. They said that only two or three of the properties were deemed habitable before the fire and were insured.

County records show that several of the houses were owned by federal agencies, such as the Veterans' Administration or the Department of Housing and Urban Development, or by mortgage companies.