Carr, at the University of Michigan, says she hopes more departments will focus on screening prostitutes, female and male, and training officers to recognize the signs of trafficking.
"Really good screening can't take place 10 minutes after an encounter with a law enforcement officer. The victim needs to be put in a safe place," Carr says.
"There are lots of incentives to not say what's happening to you."
But even when officers determine that help is needed, there's often not much they can do.
"Victims assistance is the weakest link in the chain," says Mark Ensalaco, a trafficking expert who's director of the human rights studies program at the University of Dayton.
He recalls one case, in recent years, when a young woman was rescued after an Ohio state trooper stopped a car on the interstate and recognized that she was a victim of sex trafficking. Beyond abuse, those signs can include malnourishment, having few possessions, avoiding eye contact and not having control of personal identification, such as a driver's license or a passport.
This woman, too, was addicted to drugs, Ensalaco says, but never got the help she needed. Eventually, she committed suicide.
Even in states such as Illinois, long-term help — housing, mental health counseling and trauma services that are survivor-led — are lacking, says Lynne Johnson, the policy and advocacy director for the Chicago Alliance Against Sexual Exploitation.
"We have little pockets of progress," she says, noting that much of it is aimed at minors. In Chicago, for instance, there's now a long-term safe home with space for eight girls that is funded by a private donor. A drop-in center for youth on the city's West Side, funded by federal grants, is open a couple days a week, Johnson says.