The sting was nothing new in the world of Internet policing: The 13-year-old cheerleader who found herself bombarded by sexually graphic e-mails and photographs from a 45-year-old Navy physicist turned out not to be a teen-ager at all, but an undercover FBI agent tracking online predators.

It was the kind of case typically resolved with a speedy guilty plea in Maryland's federal courts. But Navy weapons designer George P. Chambers of La Plata became yesterday the first defendant accused of using the Internet to seduce an underage girl to take his case to a federal jury in Baltimore, arguing that his fantasy life was not a crime.

Defense attorney Bryan A. Levitt of Owings Mills said Chambers never thought he was really talking to a 13-year-old girl when he traded explicit messages in an America Online chat room called "I Love Older Men." The anonymity of the Internet, Levitt said, allows virtually every user to pretend to be someone they aren't.

"The Internet has created a new society and new community with its own rules and its own way of behavior," Levitt told jurors in opening statements, in which he also suggested that the FBI sting amounted to entrapment. "They're not really patrolling the Internet. What they are doing - and I think this is a perfect case - they are luring people into situations that they never intended to get lured into."

Assistant U.S. Attorney Jonathan P. Luna said agents are patrolling the Internet in the same way Baltimore police sometimes pose as undercover drug dealers to fight the city's deadly drug problem. Investigators target child predators, Luna said, not casual Internet users engaged in virtual "dirty talk."

The prosecutors said that in more than 100 e-mails sent between February and June of this year, including ones in which court records show that Chambers e-mailed photos of male genitalia and talked about impregnating the girl, Chambers made clear he was hoping to have a sexual relationship with his young correspondent.

"The sum and substance of those e-mails were that the defendant wanted to meet her, and he wanted to have sex with her," Luna told jurors.

Chambers was arrested June 6 at The Mall in Columbia, where he had arranged a meeting with the girl. FBI agents followed Chambers as he drove about 50 miles from his office at the Naval Surface Weapon Center in Indian Head to the Howard County shopping center.

Levitt said Chambers suspected he was being tricked, purposely arriving 45 minutes before the scheduled meeting and attempting to leave the mall when he was arrested. Federal authorities said that after his arrest, Chambers blurted out: "I can't believe I came here today. My life is over."

The case against Chambers was investigated by the FBI's Innocent Images program, created in 1995 to target online pornography traffickers and stalkers. The program, based in Prince George's County, has made more than 500 arrests, flooding the federal courts with a new kind of cyberspace criminal.

Similar cases elsewhere

Other local and federal police agencies have undertaken similar efforts in recent years, and the charges have faced a number of challenges in state courts from defense attorneys who have argued that there was no crime because there was no victim - only adult investigators posing online as minors.

In federal courts across the country, however, the cases almost always result in guilty pleas without further appeals. Investigators can produce word-for-word transcripts of each online conversation, and they often are so graphic and embarrassing that few defendants are anxious to have them read aloud to a jury.

Acquittal in Miami

In one of the few similar cases to go to trial, a federal jury in Miami acquitted last spring a Florida lawyer accused of using the Internet to seduce an underage girl who turned out to be a detective working with an anti-child-pornography task force.

In the Miami case, nationally prominent defense attorney Roy Black also argued that his client was indulging in fantasy sex talk and never thought the person on the other end of the conversation really was a young girl.

Chambers, who is married and has two children, is expected to testify in his own trial, which is expected to last about two days. Levitt told the Baltimore jury yesterday that Chambers had reason to believe there were no minors in the "I Love Older Men" chat room.

Court records show that Chambers warned in one e-mail that he needed to be careful because "sometimes, cops pretend to be teen-agers online."