Prince George's County officials said yesterday that they suspect an arsonist started the fire Tuesday night that gutted the Marlboro Race Track grandstand, caused $750,000 damage and injured 13 firefighters.

The dilapidated track, known more for its role in the scandal that brought down former Maryland governor Marvin Mandel than for any races ever run there, was purchased by the county only five days before the fire. It was to be converted into a public equestrian center and parking lot.

"It's pretty suspicious to me," said County Executive Lawrence Hogan yesterday during a tour of the track. "For years it sat here, and then, a few days after the county takes title, it burns. It's my feeling it was arson."

County arson investigators believe the fire to have been of suspicious origin, but have not reached a final determination about whether it was arson. However, they said yesterday that they weren't aware of anything at the track, located along Route 4 near Upper Marlboro, that could have started the blaze accidentally.

"Most major fires that occur in abandoned, vacant building are the work of arsonists," said Ward Caddington, a Prince George's County Fire Department official who is supervising the arson investigation. Caddington said electricity at the track, which has been unused since 1973, has long been shut off, and that there were no combustible materials at the facility.

Caddington said the fire department will rule on the cause of the fire today or tomorrow.

Two of the 13 firemen injured fighting the blaze were hospitalized for smoke inhalation and heat exhaustion, but both were reported in stable condition yesterday.

If it is determined to be arson, Caddington said, investigators will question anyone who has expressed strong bitterness toward the county -- including some union members unhappy with the resolution of last month's 11-day strike.

"There's not one shred of evidence that points to any union member," Caddington said, but "just by simple nature of the fact that there's been a strike, that has to be explored. Any time there's a strike, there's bound to be hard feelings on both sides."

Caddington said guards are assigned to watch the track, but that no one was on the site Tuesday night when the fire broke out.

Hogan called the fire "a real tragedy" for the county. "We had a lot of dreams for this place," he said.

Yesterday afternon the track's grandstand lay completely gutted, its wooden seating burned away, its metal support beams tangled like spaghetti.

A motorist driving by the race track reported the fire with a citizens' band radio shortly before 9 p.m. It took about 45 minutes to control the blaze, Caddington said. Firefighters focused their energies on a successful effort to keep the fire from spreading from the grandstand -- already engulfed in flames -- to the adjacent clubhouse and other buildings.

The county purchased the track and the surrounding 150 acres -- 50 of them on flood plain -- from the Southern Maryland Agricultural Association, part of a company that also owns and operates Bowie race track.

Marlboro once was a popular and prosperous track, but by 1970 had lost much of its following. In 1971, a company with secret ties to Mandel bought the track, and the governor then used his influence to give Marlboro more racing days, increasing its value.

A short time later, Mandel's friends sold the track at a healthy profit. The former governor and five codefendants were convicted on political corruption charges in 1977. CAPTION: Picture 1, Prince George's County Executive Lawrence Hogan and fire officials inspect the damage done by a blaze at Marlboro Race Track Tuesday night; Picture 2, The Marlboro Race Track grandstand was gutted by a fire that caused $750,000 damage Tuesday night. Photos by James M. Thresher -- The Washington Post