A three-alarm fire that was deliberately set inside Fairfax County's Fort Hunt High School caused an estimated $4.5 million in damage early yesterday and closed the school for the rest of the academic year, officials said.

The blaze, which officials said was ignited in four separate places, destroyed the cafeteria, administrative offices, kitchen and computer room of the school at 8428 Fort Hunt Rd., in the Alexandria section of the county. It caused smoke and structural damage to other parts of the 15-year-old building as well.

Officials said they had no suspects in the arson, which caused what a school system spokesman called the worst school fire in the county's history.

The blaze was "a clear case of arson," the spokesman said. It was ignited in the cafeteria and in three places in the administrative office of the 1,750-student school, according to county fire investigator Richard Simpson.

"We suspect flammable liquids were used" to ignite the blaze, "but we haven't come up with any residue," Simpson said. "We're shooting in the dark."

A school system spokesman said Fort Hunt students would resume classes Thursday at Groveton High School from 12:15 to 4:15 p.m., whiel the Groveton students would go to class from 7:45 a.m. to 11:45 a.m.

Lunch services probably will not be available, but snacks will be provided, said the spokesman George Hamel.

He said lockers at Fort Hunt would be emptied and that books and other belongings would be transferred early this week to Groveton, three miles away at 6600 Quander Rd.

"Fortunately the student records were not destroyed," said county school Superintendent S. John David. "We removed them at 5 p.m."

Davis confirmed the $4.5 million damage estimate, saying "that's about what it cost to build the school." Hamel said the present value is about $8.3 million. He said all but $100,000 of the loss is covered by insurance.

According to Hamel the fire started about 3 a.m. "That's when the clocks stopped."

He said a roving security patrol had checked the school about 2:30 a.m. The school has an electrical security system and "why that didn't detect it I don't know," he said. "evidently a newspaper boy . . . saw the smoke and called in the alarm," Hamel added.

Washington Post carrier Kevin Bolin, of 8322 Stockade Dr., said he was walking past the school on his newspaper route about 4:30 a.m. when he noticed an oily smoke smell and saw a "red glow over Fort Hunt."

When he passed again, 10 minutes later, he said, he saw "flames shooting from the roof. I ran back to the house and called the fire department."

About 70 firefighters battled the blaze. One suffered minor injuries when struck on an arm by falling debris.

Hamel said a fire in the school's data processing center caused $40,000 damage three years ago. He said there had been no recent reports of vandalism at the two-story school.

In 1971 a fire caused $460,000 in damage to Fort Hunt Elementary School, 8832 Linton La. CAPTION: Picture 1, Fairfax County fire investigators move down burned-out hallway at Fort Hunt High School yesterday, left. Blaze destroyed cafeteria, kitchen and administrative offices.; Picture 2, Charred tables are visible near an entrance to Fort Hunt High School, below, where fire yesterday caused an estimated $4.5 million in damages. For Hunt Classes will be transferred to Groveton High School. Photos by John McDonnell-The Washington Post; Picture 3, A school security officer patrolling area of Fort Hunt High School fire scene walks past shards of glass that fell from now boarded-up windows. By John McDonnell-The Washington Post.