An FBI agent who posed online as an adventurous 13-year-old cheerleader testified in federal court today that a Navy physicist repeatedly sent her sexually explicit e-mails and photos this year but expressed momentary concern before they were to meet that she might be a law enforcement officer.

"Are you a police officer or do you work for any law enforcement agency?" physicist George P. Chambers wrote in an instant e-mail, read in court today by the undercover FBI agent, Emily Vacher. Vacher, who was running a sting operation, responded to him that she was not.

Chambers wrote: "good, thankyou . . . lots of cops online pretending to be girls."

Vacher kept up the ruse, writing, "they don't let 9th graders in the CIA!!!!!"

On June 6 -- one month after that exchange -- Chambers, 45, of La Plata was arrested at the Mall in Columbia on a federal charge of using the Internet to entice sex from a minor. Prosecutors said in court Monday that he was about to rendezvous with his e-mail partner.

Chambers, who has pleaded not guilty, was dismissed this week from his job as a weapons designer at the Naval Surface Weapon Center in Indian Head.

His trial, which began Monday in U.S. District Court here, provides a look into a clandestine online world where adults often approach teenagers through graphic e-mails and, in some cases, attempt to arrange meetings for sex. It also is revealing some of the tactics used by FBI agents who prowl chat rooms disguised as teenagers.

Chambers, who is married and has two preschool-age daughters, was arrested by agents working for Operation Innocent Images, an initiative started in the FBI's field office here in 1995. Since then, about 50 suspects who have traveled to meet people they believed were minors have pleaded guilty to charges, authorities said. Chambers is the first suspect charged in the operation to go to trial, authorities said.

Assistant U.S. Attorney Jonathan Luna told the jury Monday that Chambers was a predator seeking sex with someone he thought was 13 years old.

But defense attorney Bryan A. Levitt insisted that Chambers was simply expressing his fantasies. Levitt said Chambers thought all along that he was corresponding with an adult playing the role of a teenager. The defense attorney contended that there is no victim in the case and that Chambers showed up at the mall out of a "natural curiosity."

Chambers plans to testify in his defense, Levitt said.

"These are fantasies," Levitt said during a break in the trial today. "It should not be a crime to express yourself even if it's repugnant to others."

In court today, Vacher provided the jury with examples from dozens of e-mails generated since the correspondence began in February. The exchange took place in an America Online chat room called "I Love Older Men." She testified that Chambers often asked whether her parents or any adults were home while they were writing each other.

The first exchange took place Feb. 1, Vacher said. She testified that she pretended to be a cheerleader from Columbia whose parents were divorced.

"Must be hot if you are a cheerleader," Chambers wrote.

On the same day, Vacher said, Chambers offered to send her a photograph of his genitalia and then sent a graphic image. The photograph was shown to the jury today on a television monitor.

As Vacher read from some of the explicit e-mails, Chambers occasionally jotted notes and moved back and forth in his chair. Jurors read transcripts of the exchanges and at times looked over at the defense table as Vacher testified.

During cross-examination, Levitt suggested that Chambers had no intention of having a sexual encounter, pointing out that he suggested in e-mail messages that she bring along friends if she was uncomfortable.

"Didn't you think bringing a friend changed the landscape a lot?" Levitt asked.

Vacher said she didn't think it made a difference.

According to Vacher, she and Chambers kept making and canceling arrangements to meet before the June 6 meeting was set up, at a bakery at the mall.

As Chambers stood about 25 feet from the bakery, FBI agents moved in.

Robert C. Loughney, one of the agents on the scene, said Chambers told the authorities: "I can't believe I came out here today. My life is over."