Ten Prince George's County volunteer firefighters were charged yesterday with setting fires in nine vacant houses in District Heights and Boulevard Heights in the last year, allegedly for the pleasure of extinguishing them, fire officials said.

"It appears that perhaps they were overzealous in their desire to be firefighters and to engage in firefighting activity," said Prince George's County Fire Chief Steven T. Edwards. He called arson the "worst crime a firefighter could commit."

Edwards said fire investigators began making the arrests Tuesday, less than 12 hours after an administrator at the District Heights fire station discovered a diary in which a volunteer detailed the dates and locations of several arson fires and identified the people who allegedly helped set them. The last arrest was made yesterday afternoon.

Fire investigators had suspected for months that several fires in District Heights were the work of a single arsonist or a group of people who knew each other but were "absolutely nonplused" to find that members of their department were involved, Edwards said.

"I cannot tell you how appalled and distressed I am," he said. "People like this can put a black mark on the entire department and that is very, very unfortunate."

Edwards said fire investigators questioned the diary's owner, who acknowledged his involvement and implicated several other District Heights volunteers. One of them in turn told investigators that volunteers at the Boulevard Heights fire station had been involved in similar activities, Edwards said.

The District Heights fires occurred between June 1989 and last July, while the Boulevard Heights fires were set between August and December 1989, fire officials said.

"This diary essentially was a running record of the occurrences," Edwards said. "Once we had that, the whole thing began to unravel very quickly and it just kept unraveling."

District Heights Volunteer Fire Chief Thomas Stommel declined to comment on the arrests. However, fire officials said the volunteer unit had been responsible for helping investigators break the case.

Officials said they have no evidence that the fires were set for money, and they did not put a dollar value on the property that was destroyed. No one was injured in the blazes, Edwards said.

Fire investigators charged three adults and three juveniles from the District Heights station and one adult and three juveniles from the Boulevard Heights station with various counts of arson in connection with the fires.

The adults were identified as Rodney Jerome Hall, 20; Robert Wayne Vasquez, 18; Demetrius Terry, 18; and Napoleon L. Queen, 21. All of the juveniles are 17 years old, fire department spokesman Ron Siarnicki said. Queen is from the Boulevard Heights station.

Edwards said that all the suspects had undergone criminal background checks and had completed their training with the department. Most had been with the department slightly more than a year.

The Prince George's Fire Department relies on both volunteers, a longtime fixture in the county, and paid firefighters, with the proportion of each varying from station to station. Volunteers can join the department as junior firefighters at 16, officials said.

Four of the fires involved houses that were vacant but habitable, while the rest occurred in vacant houses that were deemed uninhabitable, Siarnicki said. A conviction on a charge of arson to a vacant but habitable house carries a maximum penalty of 30 years in prison, he said, while arson to an uninhabitable structure is punishable by up to 20 years.

The District Heights fires occurred in the 6600 block of Walker Mill Road, the 6400 block of Marlboro Pike, the 2600 block of Council Drive and the 7600 block of Marlboro Pike, Siarnicki said. The Boulevard Heights fires were in the 4000 block of Byers Street, the 4000 block of Vine Street and the 4300 block of Davis Street, he said.