Prince George's County officials said yesterday they have determined arson to be the cause of the fire earlier this week that destroyed the Marlboro Race Track.

The officials said that their two-day investigation eliminated all "natural causes" explanations for the fire, and as a result they have concluded that the blaze, which began late Tuesday night, was a "set fire."

They said they have no suspects in the case. Officials have offered $1,000 or more for information leading to the arrest of the person responsible for the fire, which caused $750,000 in damage to the track's grandstand.

The fire completely destroyed the grandstand of the 50-year-old track and 100 firefighters took three hours to bring it under control. Fire officials reported that 14 firefighters were injured during the fire, two of whom were hospitalized until yesterday.

The suspicions surrounding the cause of the blaze are a somewhat fitting end to the historic attack, which figured prominently in the political corruption trial and conviction of former Maryland governor Marvin Mandel.

In the last few years, the track had lapsed into disrepair, with weeds crowding its fields and its only use a parking lot for county government workers.

It once had been a posh gathering spot for county residents, but by 1979 had lost much of its following to bigger tracks such as Bowie's one-mile course.

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, thus increasing its value. A short time later, Mandel's friend sold the track at a healthy profit. The former governor and five codefendants were convicted on the political corruption charges in 1977.

Just a week ago, the Prince George's County government brought the track for $1.3 million with the intention of developing it into an equestrian center. fCountry officials said yesterday they were uncertain how the plans would be affected by the fire.

Lawrence Hogan Jr., son and top aide of County Executive Lawrence Hogan, said yesterday that the old horse stables, club house and other buildings on the 150-acre grounds of the track were not destroyed and can be used in the future.

If the county decides to rebuild the grandstand area, he said, the county would have to pay the first $250,000 in cost but insurance would cover the remaining amount.