Fairfax County officials said yesterday they suspect that last weekend's early-morning fire at Fort Hunt High School was set by insiders, possibly students, familiar with the security system.

"We have a feeling it's students, but we're not exactly sure yet," said county fire investigator Lt. Richard L. Stone. "We've received some new information, but we have no suspects as yet. We know there was more than one person involved. We're getting a lot of anonymous calls and calls from citizens. I'm sure we'll get a brak in it soon."

A $5,000 reward, offered jointly by the Fairfax school board and the Fort Hunt Parent-Teachers Association, was announced yesterday for information leading to the arrest and conviction of the persons responsible.

The fire, which caused an estimated $4.5 million in damage and forced the school's closing, was set about 3 a.m. Saturday in the cafeteria and three places in the school's administrative office, officials said.

"The person or persons involved knew how the (security) system worked and knew how to get around it," said Superinteddent S. John Davis yesterday.

Edward Cox, head of school security, said a roving security patrol had checked the exterior of the school about 2:30 a.m. and found that the building "was secure."

"I would imagine that the individual was already in the school," Davis said. He said the electrical security system was working, but because the people responsible were familiar with the system neither the break-in nor the fire was detected. A newspaper carrier reported the fire shortly after 4:30 a.m.

Davis said the security system, which is 10 to 15 years old, is being replaced with a new system. He said school officials were in the process of replacing the old system before the fire.

School officials yesterday were busy making plans to transfer the 1,712 Fort Hunt students to Groveton High School. Beginning tomorrow, Fort Hunt students will attend classes at Groveton from 12:15 to 4:15 p.m., while Groveton's 1.608 students will go to class from 7:45 a.m. to 11:45 a.m.

Dr. Herman A. Howard, area superintendent, said the double sessions will last for the next 17 days, until the semester ends. He said if school officials determine that Fort Hunt cannot be reopened this year, then the students will be divided between Groveton and Mount Vernon high schools for the remainder of the academic year.

Howard asked Fort Hunt students who generally walk to school to go to the intermediate or elementary school closest to them by 11:45 a.m., where a bus will take them to Groveton.