The increasingly common law enforcement tactic of having adult officers pose as children in Internet chat rooms to arrest potential sex offenders came under legal attack yesterday when Maryland's highest court ruled that the law does not allow the prosecution of people who merely believed they were dealing with children.

The Maryland Court of Appeals unanimously overturned the Frederick County Circuit Court conviction of Richard J. Moore, saying he could not be found guilty of committing a crime with a nonexistent victim.

The head of the Maryland State's Attorneys' Association said the ruling effectively guts efforts to catch people preying on children via the Internet.

"In terms of enforcement, with regard to computers and minors, it's difficult if you can't use undercover officers to root these people out," said Frank M. Kratovil Jr., the association's president and the Queen Anne's County state's attorney. "You could certainly continue to do it in the hopes of catching these people, but in terms of convicting them and holding them accountable for what's happening, it's not going to do it."

But Laura Rhodes, a defense attorney in Rockville, said the ruling demonstrated the court's reluctance to stray beyond the narrow confines of the current law.

"What the court is doing is simply interpreting the legislative will, and the legislature has been unwilling to cross a bright line," Rhodes said. "It's troubling to think of being punished for something that could never happen."

In explaining its reasoning, the court relied heavily on the General Assembly's intent in amending its child pornography laws since 1996. The opinion, written by Judge John C. Eldridge, noted that the legislature has tried but failed six times to broaden the law and make it illegal to proposition an adult who the suspect believes is a minor. When the General Assembly used the word "minor," the ruling said, it specifically meant "an actual person who is under the age of 18 years."

A concurring opinion, written by Judge Lynne A. Battaglia, who is a former U.S. attorney, called on the General Assembly to amend the law, perhaps along the lines of federal law and Florida statutes that have withstood judicial scrutiny.

The ruling came on a case that began in early July 2002, when Moore, a 38-year-old Elkridge man, allegedly arranged a sexual rendezvous with someone he met online who he believed was a 14-year-old girl.

In fact, Moore -- using the screen names Runner5K and Runner10K -- had been communicating with a Frederick County sheriff's deputy assigned to a State Police task force targeting Internet crimes involving children, the opinion says.

During their conversations, Sheriff's Deputy Mike Sabol assumed the identity of a 14-year-old girl and gave a detailed description of the fictitious girl, including her age. The opinion says Moore allegedly engaged in explicit conversations and expressed interest in having sex with the girl and a fictitious 14-year-old friend.

"If we were going to do this, we would have to be very discreet," Moore allegedly messaged the deputy. "The best place to go would be your apartment if your mom isn't there."

Moore allegedly arranged a meeting on July 12, 2002, in Frederick, where he was arrested. He was found guilty of violating the state's child pornography act and attempted third-degree sex offense. The trial court imposed a three-year sentence and suspended all but the time Moore already had served.