Laws cracking down on human trafficking are on the books in all 50 states, but convictions are notoriously elusive, and state prosecutors haven't come close to matching the success their federal counterparts have had in winning cases.

States need to add resources into support trafficking victims, educate the public and train law enforcement if the numbers of prosecutions and convictions are to improve, officials and experts say. In at least a dozen states, attorneys general are not even authorized to pursue human trafficking charges.

Records requested from all 50 states by The Associated Press indicate a low conviction rate since Washington became the first state to enact a human trafficking law in 2003. A previous study suggested a 45% conviction rate through roughly the first decade of the laws.

In contrast, the conviction rate for prosecutions under the federal Trafficking Victims Protection Act, enacted in 2000, is about 80%, according to Justice Department data.

"We're not fully where we need to be, but it's encouraging to see states pursue these cases," said Bradley Myles, executive director of the Polaris Project, which lobbied for passage of the state laws. "Prosecutors are still learning how to prosecute these cases successfully. We're in the process of seeing the field mature more. It's going to take time."

Underscoring the difficulties is the misdemeanor case against New England Patriots owner Robert Kraft, whose attorneys May 13 succeeded in getting video evidence suppressed . The decision, if upheld, could force prosecutors to drop charges against Kraft and potentially others among the 300 men facing solicitation charges as part of a sweeping investigation of massage parlor prostitution and possible human trafficking in Florida.

Some spa owners and operators also face felony prostitution charges, but none of the defendants has been charged under the state's human trafficking law.

Some local officials point out that prosecutors do often win convictions on other, oftentimes lower charges that can still take suspected human traffickers off the street for a time, not unlike how murder charges are sometimes downgraded to manslaughter. The study that found a 45% conviction rate also found that 72% of human trafficking cases that were examined did lead to some sort of conviction.

In the Florida prostitution case, many of the spa operators are being prosecuted under the state's Racketeer Influenced and Corrupt Organizations Act, which calls for the same maximum penalty, 30 years in prison, as for human trafficking.

That means prosecutors won't have to rely on the testimony of trafficking victims, which is frequently difficult to procure, in order to build their cases while still being able to pursue long sentences, said Jeffrey Hendriks, a prosecutor in Fort Pierce handling six of the felony cases.

"From a legal analysis, what's the loss? We want to try to put these people away for up to 30 years. Why rest your whole case on the victims?" Hendriks said. "I don't want to sound flip, but that's the analysis. It's just a better fit."

Most states aren't required to track prosecutions and convictions for human trafficking crimes.

The AP asked state attorneys general or other state agencies for tallies of human trafficking prosecutions, human trafficking convictions and convictions on other charges in their states since their local law was enacted. The AP also asked for how many cases resulted in no conviction or are still pending.

Five states did not respond. Of those that did, many supplied figures for one or some of the categories but not others, so full tallies and direct comparisons aren't possible. But the AP's review does suggest there have been many hundreds of prosecutions for human trafficking nationally, but relatively few convictions, let alone for human trafficking crimes.

At least 2,700 defendants nationwide have been charged since Washington state enacted the first law in 2003, the AP found. Only about 440 were convicted specifically of sex, labor, child or other trafficking crimes.

Nearly 500 others were convicted of lesser but related crimes, such as prostitution and drug charges. Nearly 300 others resulted in no conviction, either because of a not guilty verdict or because charges were dropped or dismissed, and more than 200 cases are pending.

Some states should consider giving their attorneys general authority to prosecute human trafficking cases, suggested Julie Dahlstrom, a law professor who heads Boston University's Immigrants' Rights & Human Trafficking Program.

State attorneys general in Connecticut, Idaho, Illinois, Indiana, Kentucky, Louisiana, Maine, Minnesota, North Dakota, Tennessee, Washington and West Virginia told the AP they lack the authority to prosecute human trafficking cases, either because primary criminal prosecutorial powers lie with district and county attorneys or because state law doesn't specifically allow them to prosecute the crimes.

But even in states where the attorney general has prosecutorial powers, convictions are still low, the AP review suggests.

In Massachusetts, at least 216 people have been charged with human trafficking crimes under the state's 2011 law, but just 18 have been convicted of them, the AP found. About 50 others were convicted of other crimes, 70 weren't convicted at all, and about 80 have pending cases.

That's a conviction rate of just over 8%.

State Sen. Mark Montigny, a Democrat from New Bedford who has proposed changes to increase the success rate, has proposed requiring the state to provide training programs for local law enforcement agencies; launch a human trafficking public awareness campaign; compile an annual report of investigations and prosecutions statewide; and designate additional public money to trafficking survivor support services.

"Sadly, these numbers are not surprising," Montigny said. "Prosecutions and convictions are unlikely to increase unless and until we enact necessary reforms."

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Follow Philip Marcelo at twitter.com/philmarcelo.