Four defendants had already pleaded guilty, and Justin Strom was about to become the fifth.

Clad in a green prison jumpsuit, Strom on Tuesday leaned forward and quietly admitted to a federal judge that he lured at least eight Fairfax girls into prostitution over six years.

Strom, 26, is a member of the Underground Gangster Crips, a Fairfax street gang. From 2006 to 2012, federal prosecutors said, he used social networking sites and false identities to coerce young girls from one of the region's most affluent communities into prostitution.

Strom and four other gang members were charged in March with running a prostitution ring that involved at least 10 girls.

All five have now pleaded guilty. Strom, who prosecutors say was the ringleader, was initially charged with 10 trafficking-related charges, but a federal judge dropped nine of those in exchange for a guilty plea on one of the most serious: sex trafficking of a juvenile, which could land Strom in prison for life.

Prosecutors said Strom solicited girls through Facebook, MySpace and Craigslist, sending more than 800 messages telling young girls they were pretty and could make money. One girl named in the criminal complaint against him was forced to give oral sex at knifepoint and made to have sex with 14 men in one night, prosecutors said.

Law enforcement authorities said they were pleased with the guilty plea, and Neil MacBride, the U.S. Attorney for the Eastern District of Virginia, said he plans to pursue a "significant sentence" for Strom when he's sentenced in September.

"My message to traffickers is that you're not welcome in Northern Virginia," MacBride said. "We will find you, hunt you down, arrest you, prosecute you and send you to jail for life if we can."

MacBride and FBI officials cautioned parents and children in the area about operations like Strom's, which prowl social media sites to entice girls into prostitution.

"It highlights the vulnerabilities of social media, even in some of the most affluent suburbs in this country," said Ronald Hosko, special agent in charge of the criminal division in the FBI's Washington field office. "It sheds light on the need for parents to be vigilant."