A Fairfax Circuit judge yesterday forbade the county Board of Supervisors to meet illegally behind closed doors, but board members said they still did not know what they did wrong during discussion of the county's land use laws last fall.

"We've been found guilty of an offense we did not commit and now we have a decision that doesn't tell us what we did wrong," said Supervisor Lilla Richards (D-Dranesville). "I don't see how we can have any further executive sessions until the court or the appeals court tells us what we did wrong."

Yesterday's order by Circuit Court Judge Lewis Hall Griffith follows a June 21 ruling he made when he was chief judge in which he ruled in favor of the Times Journal Co., which accused the board of violating state law when it met secretly to discuss proposals on limiting development.

The order is to expire when the current board's term ends in December 1991.

Griffith ruled that the board met illegally in executive sessions on Nov. 28, Dec. 4, Dec. 5 and Dec. 11 while it considered reducing development permitted on about 14,000 acres zoned for commercial and industrial uses.

Those meetings resulted in a new county amendment passed by the board Dec. 11, significantly reducing zoning throughout the county. Some landowners were outraged by the amendment and 269 lawsuits were filed challenging it. Many of those suits have been consolidated and a trial in those cases has been set for Sept. 25.

The order entered yesterday said that "those executive meetings were devoted at least in part to matters that exceed what is allowed under the provisions" of the Virginia Freedom of Information Act.

That law requires meetings of public bodies to be conducted in public except under certain exemptions, including consultation with lawyers about actual or probable litigation.

Richards said that during those sessions board members were discussing probable litigation about the proposed zoning amendment.

"We were advised by five different lawyers. We were asing questions of the county attorney about the impact of different provisions in the grandfather clause on the lawsuits that were probable," Richards said.

The Journal alleged in its suit that the board was trying to reach a consensus on the proposed ordinance, citing a comment made by board Chairman Audrey Moore about reaching a consensus. Moore declined to comment yesterday.

"I have done nothing wrong and neither has anyone else on the board, except Mrs. Moore used the wrong word," Richards said. "If I'm going to be found guilty of something I did, I should have a chance to testify in my own behalf. I want this very unspecific order appealed."

Martin Wald, an attorney for the Journal, said yesterday that the Journal had offered an order with specific findings but that the county attorney objected.

At a hearing two weeks ago, County Attorney David T. Stitt told the judge: "What the Journal has done is stuck specific findings in the order the court did not make."