An administrative officer for the Virgina state parole board said yesterday that the board acted hastily and may have been misinformed when it recently freed two Fairfax County teen-agers who were serving one-year sentences in the Fort Hunt High School arson case.

Jean Anderson, who is in charge of implementing the board's policy actions, said in an interview that the decision to release Robert P. Smithwick, 18, and Timothy M. Greer. 19, last Friday after they served 3 1/2 months of their sentence was "possibly wrong." She blamed the decision on a new Virginia parole law that she said "does not answer a lot of questions."

But, she said, state officals are powerless to reverse the releases even though complaints about the action from Fairfax County prosecutor Robert F. Horan Jr. may be valid. Horan complained earlier this week that the paroles were "really crazy" and "terrible misleading" to the public.

Yesterday Horan followed up his initial charges with a letter to the parole board arguing that its actions were wrong because the board counted time Smithwick and Greer spent in the county jail instead of time they had spent in a state prison. The prosecutor said that the new law allows the early release of people who have been committed to a state correctional facility, not a locally operated jail.

"This case simply doesn't come within the mandate of that statute," Horan said yesterday. "Neither (Smith-wick nor Greer) were even in a state facility," he said. "They never left the Fairfax County jail."

In Richmond, Anderson agreed Horan may be correct. "I know exactly what he is talking about," she said, "The statute really doesn't say whether it applies to a local jail or not."

Horan has complained that the new law, which generally shortens the lengths of prison sentences, makes it difficult for prosecutors and judges to calculate when a prisoner would be eligible for parole. He also said that the parole authorities may have erred by counting as "good time" the time the two Fairfax youths spent in the county jail.

The two youths were initially jailed when charged with arson and spent 109 days in jail before they were released on bond. They later were arrested on other charges and spent another 30 days in jail after an incident at the home of a Fort Hunt school principal.

Smithwick and Greer were convicted of misdemeanors in incidents unrelated to the arson.

Greer was found guilty by a jury in the Fort Hunt case and Smithwick pleaded guilty to the same charge, malicious burning. Each was sentenced to one year in prison. They also were ordered to make $10,000 financial restitution to the county over a 10-year period, and to perform 3,000 hours of volunteer community work within a two-year period. Neither the financial restitution nor the volunteer work parts of the sentence were affected by the parole decision.

Anderson said she received release forms on Smithwick and Greer only 10 minutes before the parole board office in Richmond was to close on Friday. She said she "didn't know until Monday or Tuesday who Smithwick and Greer were.

W.K. Cunningham Jr., the only parole board member in the office at the time, signed the papers and Smithwick and Greer were freed immediately, Anderson said.

Assistant State Attorney General Linwood Wells, who advises the parole board, agreed yesterday that the new law is vague but said he "failed to see the significance" of Horan's question of whether Smithwick and Greer had served time in a local or state correctional facility. He said it was irrelevant where they served their time.

Wells said the release were products of a law he described as "a two-or-three-sentence statue that attempts to invoke a brand new system of parole." He said the law "is stated in such broad terms that several problems of interpretation have occurred, like the case [Horan] brought up."

Matthew Musolino's 18, the third youth charged in the arson case, is due to be sentenced August 3 on a misdemeanor charge of destroying private property. Horan suggested yesterday that Musolino's situation had been "tremendously altered" by the parole board's decisions, but he refused to elaborate. CAPTION: Picture, ROBERT F. HORAN JR. . . . complaints on parole law