Authorities yesterday added two recent arson cases to a list of fires they suspect were set by a serial arsonist in the Washington area during the last 18 months, bringing to 44 the number of cases being investigated by a regional law enforcement task force.

Kelly Long, an agent of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said the two recent cases -- a fire at a Wheaton condominium building early Thursday and a car fire two weeks ago in Northeast Washington -- were similar to the others suspected of being set by a serial arsonist.

"The evidence found at the scenes fits the same criteria as the other arsons, including the time of day and the flammable liquid used," Long said at a news conference. She said three others fires, which had been tentatively linked to a serial arsonist, are now conclusively linked. One of those three fires occurred Aug. 30 in Fairfax County, and two were set Sept. 17 in Prince George's County and the District.

In most of the cases, small fires have been set at apartment buildings or houses late at night, using an accelerant. The car fire was the first of its kind added to the list. Authorities said a vehicle parked in the 2800 block of Channing Street NE was torched early Sept. 8 in an area that is a short walk from the 2800 block of 30th Street NE, where a fire linked to the arsonist was set Monday.

The Wheaton fire, in the 10800 block of Amherst Avenue, was set Thursday.

"Everything about it fits," Long said of the car fire and the Wheaton fire.

Including the two recent fires, 43 arsons and one attempted arson have been linked to a serial arsonist since March 2003. They have occurred in the District and Alexandria and in Prince George's, Montgomery and Fairfax counties. One fire, in June 2003, killed an elderly woman in the District. Several people have been treated for minor injuries at other fires.

In most cases, an accelerant was spread outside the homes or apartment buildings, often near a door. Also, plastic jugs with a cloth wicks and traces of a flammable liquid have been left behind at nearly all the scenes.

Other patterns were mentioned at yesterday's news conference. Only two fires linked to the serial arsonist were set in July or August of this year or last year, an indication that the arsonist might leave the region during the summer months.

"Maybe he's a student, or maybe he goes on vacation every year at that time," Long said.

A psychological profile of the arsonist completed last year describes the suspect as someone with a chameleon-like personality who is insensitive when angry but can behave in a caring way when necessary. The person also is likely manipulative and a skillful liar who shuns responsibility.

Serial arson, according to the profile, is a means of stress relief, so the fire might have been set during especially turbulent periods in the arsonist's life, the officials said.

"It is very frustrating," Long said.

She urged people to remain vigilant.

"Keep your porch lights on, make sure you don't have combustibles near your house and be aware of unusual people in your neighborhood," she said, adding that anyone with information should call the task force's tip line at 301-77-ARSON.

Federal agent Kelly Long said at a news conference that the suspect might leave the area during summer.