She was a month shy of her 17th birthday when the gang’s leader approached her outside a Tennessee hotel, engaging her in playful banter. But soon “Chyna,” as she would come to be known, was traveling up and down the Eastern Seaboard with the gangster and his friends, having sex with strangers for cash.

At a trial in federal district court in Alexandria this month, prosecutors said Chyna was one of at least four teenagers sold as sex workers by the “Cold-blooded Cartel,” a Georgia-based gang that found Northern Virginia to be an especially lucrative territory to prostitute children. Their case, prosecutors say, is indicative of a disturbing phenomenon in the Washington area. More and more gangs are prostituting teens, finding it a bigger moneymaker than drugs or weapons.

“The profit margin, the income stream that comes from selling the bodies of children . . . is enormous,” said then-U.S. Attorney Neil MacBride in an interview before he left the post this month. “Sort of like capitalization welcomes all comers to the marketplace, criminal activity is the same way.”

Federal prosecutors in the Eastern District of Virginia have recently handled several cases involving gang members and juvenile prostitutes. A leader of the Cold-blooded Cartel was sentenced to 25 years in prison in June, before the trial this month of two others involved in his prostitution ring. A member of the Mara Salvatrucha street gang, also known as MS-13, was charged last month and accused of helping others in what prosecutors called an “underground prostitution business” involving four female runaways.

The office has prosecuted 27 defendants in gang-related trafficking cases since 2011 and at least 28 juvenile victims have been identified, authorities said.

“This is an emerging trend, and the gangs, we think, have made a horribly wrong business calculation that they can make a lot of money and get away with it,” MacBride said.

Prosecutors say that members of the Cold-blooded Cartel plied the girls with marijuana and ecstasy to make them more receptive. The gang exerted control through a combination of violence — at least one member carried a gun — and affectionate overtures. Two of the girls had tattoos of a gang leader’s nickname, “Boo.”

Prosecutors and advocacy groups say that MS-13 seems to have pioneered juvenile sex trafficking as gang business — even stepping away from traditional violence and turning to prostitution. MacBride said the endeavor is especially profitable because girls, unlike drugs, can be sold more than once, and the costs to recruit and prostitute them are minimal.

The concern, though, is that other gangs have started to follow MS-13’s lead, and they have started using more sophisticated online recruitment through social networking sites. Last year, a man affiliated with the Underground Gangster Crips was sentenced to 40 years in prison for running what prosecutors described as one of the largest underage prostitution rings in the Washington area’s history. And that apparently did not deter the Cold-blooded Cartel.

By prosecutors’ account, the group prostituted women and high school girls in at least seven states, including Maryland and Virginia. It advertised services on the Web and had girls meet clients at hotels including the Homestead Studio Suites in Sterling, the Aloft hotel in Ashburn and the Washington Dulles Marriott Suites in Herndon.

Chyna testified this month that she advocated moving the operation to Northern Virginia, saying Herndon was the most lucrative place she ever worked.

“There is a lot of money here,” she said.

Experts say it is difficult to pinpoint the number of children trafficked for sex in the United States, and it is even more difficult to determine how many are trafficked by gang members.

Bradley Myles, chief executive of the advocacy group Polaris Project, said his organization responded to more than 2,700 cases of child sex trafficking in the past five years stemming from calls to a national hotline it operates for victims and tipsters to report suspected human trafficking. Calls addressing all types of trafficking have increased from about 400 a month in 2007 to about 3,000 a month now, but it is unclear whether that is because trafficking is more prevalent or just better publicized, Myles said.

Melissa Snow, a child sex trafficking specialist at the National Center for Missing and Exploited Children, said her organization estimates that one in eight “endangered runaways” in 2012 were likely sex trafficking victims, and officials are “certainly seeing that gangs as a trend are expanding to include child exploitation or child sex trafficking.”

That is especially troubling, experts say, because of the control gangs are able to exert over children through fear and ma­nipu­la­tion.

Chyna, who wore khaki pants and a plaid shirt in court, bit her lip as she testified. She told jurors that she followed strict rules, such as “Get the money upfront,” and “Don’t tell nobody my age.” And when Fairfax County police sent an undercover officer to the Hyatt House hotel in Herndon in a December sting, Chyna testified that she insisted others make sexual contact with the man they suspected was working undercover, just as his colleagues burst into the room.

“I’m like, ‘One of y’all need to touch him,’ ” Chyna testified. “ ‘I need to see y’all touch him.’ ”

At the time of the trial, Chyna said she was living in Indiana.

The issue has drawn some attention on Capitol Hill, where Rep. Frank R. Wolf (R-Va.) has pushed the attorney general to provide guidance on prosecuting Web sites that allow pimps to advertise girls. And though prosecutors have not launched efforts to shutter the sites, they have been especially aggressive in Northern Virginia in going after site users.

After two leaders of the Cold-blooded Cartel pleaded guilty to engaging in a child-exploitation enterprise — a serious and hard-to-prove charge that carries a 20-year mandatory minimum sentence — prosecutors asked that they spend life in prison. One was sentenced to 25 years, the other to 20.

And prosecutors pursued the same charges against two much-lower-level members of the ring — a woman whose attorney said was also being prostituted and her boyfriend, whose attorney said was not part of the juvenile prostitution enterprise. A jury convicted them of sex trafficking of a child, an offense that carries a 10-year mandatory minimum sentence.

MacBride, the former U.S. attorney, said prosecutors in Virginia might not bring the most onerous charges in every case, but they are aggressive in prosecuting those who sell children to try to keep the “emerging” phenomenon from becoming “systemic.”

“It’s one thing to sell a bag of drugs,” he said. “It’s a whole different deal when you’re selling the body of a 12-year-old.”