When FBI agent Doris Hepler goes undercover, there are no wigs, fake eyelashes or hidden microphones. Her identities are defined by a few deft taps on a computer keyboard in a Landover office building.

On a recent afternoon, the 33-year-old agent became Tiffany, a 13-year-old from Miami who enjoys biking and jogging and whose favorite quote is, "If it feels good, DO IT." Her mission: enter the community, strike up bawdy conversation and get propositioned by older men.

The community in this case was a "chat room" on the America Online computer service. Less than 15 seconds after she entered, Hepler was in full form.

"Are you Horny??????" flashed a message from a user named Charlie.

"What does horny mean?" responded Tiffany.

After a bit of steamy conversation, Charlie asked Tiffany if she wanted to trade pictures of young children.

Within five minutes, a digitized picture from Charlie popped into Hepler's electronic mailbox, a fuzzy color shot of two nude, prepubescent girls kneeling on a bed. After satisfying herself that the girls were underage, Hepler carefully saved the messages and the image on her computer's hard disk. It would be entered into a fast-growing evidence file at the end of the day.

Hepler and seven other agents in the Landover office spend most of their working time on the dark side of cyberspace, coordinating a nationwide federal investigation into on-line pedophiles and child pornographers. In September, their work resulted in 15 arrests and the searches of 125 suspects' computers, which FBI officials say could lead to more arrests and indictments in the next few months.

But the two-year-old investigation, dubbed "Innocent Images," is not without controversy. The agents' undercover techniques, especially their use of suggestive user names and raunchy on-line conversation, have been criticized by some civil libertarians and computer groups.

The operation is part of an expanding effort by law enforcement agencies to catch up with the growing world of computer crime. While the majority of computer-related misconduct is still business crimes such as hacking, in which computer systems are invaded and sabotaged, and money laundering, law enforcement officials say criminals are increasingly using computers for crimes against people, such as making bomb threats and setting up sexual encounters with children.

Kevin Stafford, the agent in charge of the Landover squad, said the office gets at least two or three complaints a day about on-line child pornography and messages seeking sex with minors. The FBI does not keep statistics on such crimes, and Stafford acknowledged that the material is a small fraction of all communication on the Internet and commercial networks.

"Not everybody will get dirty pictures of children sent their way," Stafford said. "But some will, and we need to stamp that out."

While Congress spent much of last year debating legislation to penalize adults who transmit "indecent" material to children over computer networks, the Landover agents have used a variety of existing laws against child pornography and the solicitation of minors to justify their searches and make arrests.

Stafford said the squad strictly limits its pornography investigations to large distributors. Images must clearly depict minors, and they must be explicit. The only arrests for solicitation have been of individuals who arrange and show up at face-to-face meetings, he said, and the squad only responds to complaints about certain chat rooms or users. "We don't go surfing the 'Net looking for this stuff," he said.

But that hasn't kept the agents from creating suggestive user profiles and engaging in steamy electronic conversations.

For example, the user name that Hepler chose for Tiffany, which the FBI asked not be revealed because of ongoing investigations, clearly indicates that she enjoys trading pictures of young children.

And when an agent pretending to be a 14-year-old girl and using the name "One4fun4u" was approached for sex by a 31-year-old Arlington patent lawyer in the summer, the agent wrote that she had been with an older man before and that "it was a lot of fun."

The two eventually set up a meeting at Montgomery Mall in Bethesda on a Saturday afternoon. When the lawyer, James F. Childress, arrived, he was arrested by federal agents. A U.S. District Court jury found him guilty in November of one count of crossing state lines for the purpose of engaging in sexual activity with a minor. He is awaiting sentencing.

In the case of William B. Bomar, a 58-year-old Bowie resident who pleaded guilty in November to the same charge, court documents detail how agents used the names "Horny15bi" and "Sexcollctr." The names had on-line profiles that listed "dreaming of kinky sex" and "sex with men" as hobbies. Horny15bi's profile included the quote, "Vice is nice but incest is best."

Bomar initially found the "girls" on-line after searching for user profiles with the word "sex." Although he sent the first message and several subsequent ones that were sexually explicit, the "girls" responded with suggestive conversation.

"Please tell us exactly what you will do to us if we both like you and we agree to have sex," Sexcollctr wrote in one message. "We have lots of ideas where we could do it."

Bomar eventually was arrested when he arrived at a McDonald's in Vienna for what he thought would be an encounter with the girls. He was holding a bag containing a camera, vibrators and condoms, according to the FBI.

Electronic privacy groups question whether the squad's undercover techniques are overzealous.

"Are we making the world a better place by tempting some of these people to commit crimes they may not have otherwise committed?" said David L. Sobel, staff counsel for the Electronic Privacy Information Center, a Washington-based on-line civil liberties group.

Legal experts say that the FBI's conduct does not appear to constitute illegal entrapment but that such issues likely will come up as suspects nabbed in investigations face trial.

"Sending racy messages like they did isn't against the law," said Paul Marcus, a law professor at William and Mary and a leading specialist on entrapment law. "But it does raise the question of whether this is a wise use of government resources."

Stafford defended the bureau's tactics, saying that agents usually wait for suspects to initiate sexually explicit conversations.

"We're trying to fit in with the language and activities in some of those {chat} rooms," Stafford said. "The bottom line is that children are being abused . . . and clearly these individuals we're identifying through this investigation are a threat to society."

The unit got its start in 1993, when federal agents began probing the disappearance of 10-year-old George Stanley "Junior" Burdynski from his Prince George's County neighborhood. That investigation led Stafford and Hepler to two local pedophiles who would send electronic messages to boys on a private computer bulletin board service.

The FBI unit now has more than $1 million worth of equipment, including high-speed modems and large data storage devices, and plans to move to a larger office this month.

Squad members take turns using the office's computers, choosing from a list of aliases posted on the wall above the machines. They often spend evenings and weekends on-line, the times when children are most commonly logged in.

In cases where suspects' computers have been seized, the agents have run up against password-protected and encrypted files, which sometimes have taken FBI technicians hundreds of hours to decode.

Stafford said the squad likely will expand, and Washington and Baltimore area police departments are mulling over his proposal to create a regional task force on on-line child sex crimes.

"We're plowing a lot of new ground for law enforcement," Stafford said. "Everything is growing in cyberspace, including crime, and we're trying to keep up with it." Staff writer John Schwartz contributed to this report. CAPTION: FBI agent Doris Hepler monitors on-line networks from an office in Landover. She and seven other agents are involved in an undercover, nationwide federal investigation into on-line pedophiles and child pornographers.