The department’s flip interviews almost always failed, and even if they worked, there was no place to put the girls to receive treatment. Officers resisted investigating what they viewed as a nuisance, not a crime. Prosecutors regularly refused the cases against pimps because the girls made for shaky witnesses and unsympathetic plaintiffs.

Frustrated with this system, Sergeant Fassett started combing through old case files, looking for patterns. One stuck out: 80 percent of the prostituted children the department had handled had run away from home at least four or more times a year.

“It dawned on me, if you want to effectively deal with teen prostitutes, you need to look for repeat runaways,” he said.

In 2005, Sergeant Fassett created the “High Risk Victim” unit in the Dallas Police Department, which flags any juvenile in the city who runs away from home four or more times in a given year. About 200 juveniles per year fit that description. If one of those children is picked up by the police anywhere in the country, the child is directed back to Sergeant Fassett’s unit, which immediately begins investigating the juvenile’s background.

The unit’s strength is timing. If the girls are arrested for prostitution, they are at their least cooperative. So the unit instead targets them for such minor offenses as truancy or picks them up as high-risk victims, speaking to them when their guard is down. Only later, as trust builds, do officers and social workers move into discussions of prostitution.

Repeat runaways are not put in juvenile detention but in a special city shelter for up to a month, receiving counseling.

Three quarters of the girls who get treatment do not return to prostitution.

The results of the Dallas system are clear: in the past five years, the Dallas County district attorney’s office has on average indicted and convicted or won guilty pleas from over 90 percent of the pimps arrested. In virtually all of those cases, the children involved in the prostitution testified against their pimps, according to the prosecutor’s office. Over half of those convictions started as cases involving girls who were picked up by the police not for prostitution but simply as repeat runaways.

In 2007, Congress nearly approved a proposal to spend more than $55 million for cities to create pilot programs across the country modeled on the Dallas system. But after a dispute with President George W. Bush over the larger federal budget, the plan was dropped and Congress never appropriated the money.