It began with teachers and counselors becoming suspicious.
The same teenage girls were often absent from their classes on Fridays and Mondays. When they came to school, they wore flashy jewelry and provocative clothing and often carried more than one cellphone. Sometimes they had visible bruises.
From those observations came an investigation by the FBI and the Sheriff's Department that led this month to the indictment of 22 suspected gang members and associates on charges of running a multi-state prostitution ring, including seducing underage girls into "the life."
The defendants recruited about 100 girls and young women by promising a lavish lifestyle and emotional support and plying them with alcohol and drugs, federal prosecutors said. When those tactics failed, the pimps used violence, they added.
The operation was centered in Spring Valley and Lemon Grove in eastern San Diego County, according to the indictment. Many of the underage girls attended schools in the Grossmont Union High School District.
Similar indictments in recent years have targeted gangs in Oceanside and the North Park neighborhood of San Diego. More indictments linking gangs and prostitution are anticipated by the U.S. attorney and district attorney.
With drugs and weapons becoming riskier business ventures because of more aggressive law enforcement, gangs have moved into prostitution. The money is easy and the Internet and social media have replaced streetwalking and red-light zones as an effective method of attracting johns, officials said.
"It's absolutely an epidemic, not just in San Diego but across the country," said Robert Howe, assistant special agent in the San Diego office of the FBI.
The San Diego region, for reasons that are unclear, appears to be a dubious leader in this new trend.
A study this year of eight cities by the Washington-based Urban Institute found that, of the eight, San Diego has "the most amount of gang involvement in the underground commercial sex industry."
The study also found that 20% of the prostitutes recruited by gangs are underage and that gangs have developed "a certain level of business savvy and acumen."
The recent indictment alleges that rival gangs that normally use violence to settle disputes over turf and drugs have decided to cooperate in running a prostitution operation. Different gangs have different responsibilities: transportation, recruitment, marketing, etc.
Not satisfied with just being local, the "hybrid gang" was eager to expand its market to other states. Unlike selling drugs or weapons, there is no need in a prostitution business to keep a costly "inventory," and there are plenty of potential employees and customers.
"It's a grotesque version of a legitimate business model," said U.S. Atty. Laura Duffy.
Duffy last week talked to principals and teachers at Grossmont to thank them for their help and also ask that they redouble efforts to watch for telltale signs that gangs are recruiting their students. Also, students should be warned about pimps.
"We have to be dialed in like never before," Duffy said.
With a grant from the U.S. Department of Education, the Grossmont district has developed a guidebook for parents and educators, "Human Trafficking in America's Schools." There are discussions about a possible national roll-out of the guidebook next month by high-level officials in Washington.
There are already websites advising would-be pimps on how to apply psychological pressure to persuade girls and women to become prostitutes.
One difficulty in prosecuting prostitution cases is lack of cooperation from the victims. Some will insist they decided to sell sex only to get money to help their boyfriend launch a rap music career.
"They do not self-identify as victims," said Summer Stephan, chief deputy San Diego County district attorney. "They have been so traumatized, they want to believe this is a choice they've made willingly. They live in an altered reality."
According to the recent indictment, 19 men and three women were part of a hybrid gang called Tycoons that sent girls and young women to customers in California, Texas, Arizona, Kansas, Michigan and Nevada.
The Tycoons are also accused of attempted murder, drug trafficking, burglary and robbery, according to the indictment. The 22 face federal charges of conspiracy to engage in racketeering, which can carry a life sentence.
Tycoons members boasted of their prostitution business on Facebook and in freestyle rap songs about prostitution and drugs on YouTube, according to the indictment. Prostitutes were often "branded" with gang tattoos to show they belonged to a specific pimp.
One of the female defendants, who called herself Christal Cash, warned on a Facebook posting that any prostitute who refused to turn over money to her pimp would be beaten, according to the indictment. One defendant allegedly tried to enroll one of his prostitutes in high school to act as a recruiter.
Tycoons members allegedly split into subgroups with names such as Tycoons/Additup and Play Girl Fantasy. Hats and shirts often carried the name of their subgroup, according to the indictment.
A common place to recruit potential prostitutes was the trolley station in Lemon Grove, a gathering spot for young girls, including runaways.
"Gang members are exploiting their dreams and stealing the souls of children," Duffy said.