A Navy physicist, accused of soliciting sex from an undercover FBI agent posing online as a teenage cheerleader, testified in federal court today that he was simply acting out a fantasy in cyberspace and thought all along that the purported cheerleader was actually an adult or even a member of law enforcement.

"If I thought she was under 18, I would not continue," said physicist George P. Chambers, 45, of La Plata.

Chambers exchanged e-mails with the undercover agent for four months before he was arrested June 6 at the Mall in Columbia, where authorities said he attempted to rendezvous with his e-mail partner. His testimony came on the third day of his trial in U.S. District Court here on a charge of using the Internet to entice sex from a minor.

Chambers told the jury of nine men and three women, "I didn't think I was doing anything wrong."

Jurors deliberated about 2 1/2 hours tonight, and deliberations were scheduled to resume Thursday.

After the jury left his courtroom to begin its work, Judge Andre Davis described the case as "a surreal experience for me." He also called it "a very sad case, certainly one of the saddest cases I ever presided over."

The trial has provided a glimpse into the perils of the Internet and the clandestine world where some adults seek sexual encounters with minors. Chambers was targeted by agents working for Operation Innocent Images, an FBI initiative meant to thwart people who engage in online child pornography or child sex exploitation. Unlike most suspects arrested by the FBI in such cases, Chambers chose to go to trial.

Prosecutors have portrayed Chambers as a sexual predator, saying that dozens of e-mails, including graphic photographs, provided a chronicle of his intentions.

During roughly an hour of testimony, Chambers gave jurors a list of reasons why he said he thought the person he was corresponding with was an adult. He said that he believed parental controls would prevent minors from entering the America Online chat room he was using, known as "I Love Older Men." He also testified that the language used by his e-mail partner appeared to be that of an adult, not a young teenager.

Chambers told jurors that he knew of many women online who fantasized about being virgin teenagers. "It seems to be a very popular fantasy among women," he testified.

He tried repeatedly to arrange telephone contact with the purported teenager, Chambers testified, but she continually declined, making him suspicious of her age and intentions. He said he found it "incongruous" that she would not talk to him on the telephone but was expressing a willingness to meet and have sex.

Chambers said he went to a bakery at the mall -- a prearranged meeting place -- out of curiosity and expected to meet an adult.

He said he was originally drawn to communicate because of the idea that he was corresponding with someone posing as a cheerleader. "It's kind of a male fantasy."

"Is it your fantasy?" defense attorney Bryan A. Levitt asked him.

"Sometimes," he said.

Chambers, who has been placed on unpaid leave from his job as a weapons designer for the Naval Surface Weapon Center in Indian Head, is married and has two preschool-age daughters. He said he found escape in online sexual talk, adding, "It's a way of being anonymous, interacting with other people who might have a similar fantasy."

Among the photos he sent, Chambers said, was a 10-year-old picture of himself, shirtless and muscular. He said that it is common for people to transmit photographs that don't accurately represent themselves and for people to lie online.

He testified that he was concerned that he might be corresponding with an FBI agent because he was aware that authorities monitor the Internet. But he said he did not believe he could be arrested for simply walking around a shopping mall.

In closing arguments, Assistant U.S. Attorney Jonathan Luna countered that Chambers had driven to the mall from his workplace in Indian Head, a 60-mile drive, to go beyond fantasy. Luna said the evidence was overwhelming that Chambers believed he was dealing with a teenager.

Levitt described his client as a "nice guy with a crazy twist in him. He likes to get online and express himself in a vulgar way." He urged the jury to acquit Chambers to protect the rights of people to express themselves.

"We all have a dark side," Levitt said. "This is the way Mr. Chambers expresses his fantasies."