We've learned a lot in recent years about the growing, heinous crime of sex trafficking. Traffickers target the most vulnerable victims they can find — those who are hurting and isolated, without strong bonds to family or other would-be protectors.

Teens in residential treatment centers would be obvious targets for such predators. They've been placed in RTCs because they've been through hell — unsafe homes, abuse of all sorts, violence all around them, involvement in sex trades. Most need intensive supports for recovery, but still "act out" in ways that are dangerous to themselves and others. It's enough for traffickers to smell blood.

So how could the myriad agencies charged with supporting these teens miss a trafficking ring that exploited girls in the child welfare system, including at least nine who were at Hawthorne Cedar Knolls, a now-closing RTC in Mount Pleasant? These girls, as young as 13, were "recruited" by traffickers and coerced into prostitution, federal authorities said Thursday. Eight indictments cover crimes allegedly committed between 2010 and 2018, a long period of time to go without detection.

Minors living at RTCs are not supposed to be so isolated or vulnerable. They've been placed, in most cases, by the New York City Administration of Children's Services. RTCs are overseen by the state Office of Children and Family Services, which is involved in numerous efforts to stop trafficking and identify victims. Another state agency, the Justice Center for the Protection of People with Special Needs, was set up in 2013 to watch out for people at state-licensed residential settings like RTCs.

The young resident of RTCs, as wards of the state, are supposed to have some degree of protection.

We all know that government agencies that deal with troubled youth tend to be underfunded, understaffed and ignored — except when something goes wrong and the children in their care make headlines. But in an egregious case like this, someone needs to figure out what went wrong and take steps to prevent the most vulnerable children from being further victimized as sex slaves.

It would seem to be the very mission of the state Justice Center for the Protection of People with Special Needs to figure out what steps need to be taken to reduce the likelihood of traffickers reaching, grooming and exploiting youth who are in the child welfare system.

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If convicted, the 19 defendants, all from New York City, face sentences of 10 or 15 years to life in prison.

Hawthorne Cedar Knolls has been closing in stages since the summer, relocating its residents to New York City. Its final six children are supposed to be out by Tuesday. The nonprofit that operates the facility, the Jewish Board of Family and Children’s Services, had faced intense criticism from the community and politicians for being unable to prevent residents from leaving the campus and sometimes committing crimes.

RTC residents are allowed to leave their campuses, but are supposed to seek permission from the state, a cumbersome process. Not surprisingly, considering why they are there in the first place, they often leave without permission, usually to head home to NYC. Police might report them as runaways, but could not stop them. This freedom to come and go helps explain why residents could be vulnerable to traffickers, but not how so many residents could be victimized over so long a period of time, apparently undetected by the people charged with helping them.

The reality is that the most at-risk kids in New York have few advocates, other than the public agencies responsible for them and the nonprofits that house and treat them. They're traumatized and adrift. Few Westchester residents know that they're here and even fewer know why.

But we should all be aghast that these girls, who have lived among us, cared for by the government agencies that represent us, could be victimized so terribly, for so long.

Gary Stern is engagement editor. Twitter: @garysternNY