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BOSTON — They're nestled amid bustling downtowns and tucked into nondescript strip malls across quiet suburbs. Brothels posing as massage parlors and Asian spas have been part of many American communities for decades, hidden in plain sight.

But the Florida prostitution sting that ensnared New England Patriots owner Robert Kraft this month is a reminder of the human trafficking and abuse taking place behind the darkened windows of many of these storefronts — and how challenging the problems are to address.

Kraft is not alleged to be involved in human trafficking.

The case also highlights how police and prosecutors use an increasingly broad range of approaches, including deeper investigations into wider criminal networks, crackdowns on online sites where johns trade detailed sex reviews and enforcement of stricter civil codes on the massage industry, anti-trafficking activists said.

"You're fighting against a multibillion-dollar industry that's very, very good at being strategic and keeping their business going," said Stephanie Clark, executive director at Amirah, a nonprofit that runs a safe house for women escaping sex trafficking in Massachusetts, where illegal massage parlors have proliferated. "They are always 10 steps ahead."

As many as 9,000 illegal massage parlors currently operate in more than 1,000 cities nationwide, fueling a more than $3 billion industry, according to the Polaris Project , a nonprofit that runs the National Human Trafficking Hotline.

Most of the prostitutes are women from China and South Korea in their mid-30s to late 50s who have entered the country illegally, are deeply in debt and are drawn into sex work through a combination of lies, threats and other forms of coercion, the organization said.

The massage parlor in Jupiter, Florida, where Kraft, a 77-year-old Massachusetts billionaire, was allegedly videotaped engaging in sex acts is typical of the model.

A spokesperson for Kraft has denied he "engaged in any illegal activity." Authorities have charged him with two counts of misdemeanor solicitation.

Tucked into a pedestrian strip mall in an affluent oceanside community, the Orchids of Asia Day Spa employed mostly immigrant workers and was linked allegedly similar operations in other Florida counties, authorities said.

Authorities say the prostitutes averaged about 1,500 clients a year, were given no days off and were not allowed to leave the site, where they also lived. Palm Beach State Attorney Dave Aronberg described it as "modern-day slavery."

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Eleven alleged owners and site managers face a range of prostitution-related offenses. At least one, 49-year-old Lan Yun Ma, of Orlando, faces human trafficking charges. Hundreds of male customers, including Kraft, also face minor soliciting prostitution violations.

"We need to get beyond the whack-a-mole strategy of taking out one retail location at a time," said Bradley Myles, Polaris' CEO. "We need to see multi-state investigations that take a longer look, follow the money and build these organized crime cases."

Law enforcement officials in California, which is home to roughly a third of the nation's illegal massage parlors, as well as jurisdictions in Minnesota, Utah and Washington are also landing similar large cases, Myles said.

In Massachusetts, about half of the more than 50 people charged under the state's 8-year-old anti-human trafficking law were involved in illegal massage businesses or residential brothels, according to state Attorney General Maura Healey's office.

In one recent case, a 38-year-old woman was charged with running a human trafficking and money laundering operation across six Asian massage parlors in the suburbs north of Boston.

Prosecutors say Xiu J. Chen recruited Asian women from New York and arranged their appointments, transportation and housing, where they typically slept on mattresses on the floor. Chen was sentenced to five years in prison in December.

But in New York, another major hub of the illegal massage parlor industry, major busts involving sex traffickers remain frustratingly elusive, said Chris Muller of Restore NYC, a nonprofit that works with immigrant sex trafficking survivors.

That comes despite police rolling out a new human trafficking strategy in 2017, promising to crack down on customers, pimps and brothel operators rather than arresting prostitutes. New York police emailed statements about their new approach, but they did not provide arrest data or comment on why they did not appear to have made major arrests of traffickers.

Muller said a silver lining is that prostitute arrests have dropped nearly 50 percent while the arrest of johns has spiked nearly 200 percent. Authorities, he added, are also helping connect more prostitutes with groups like Restore NYC that can help get them on a path to citizenship and break the grip of traffickers, who oftentimes hold their passports and immigration documents as collateral.

New York is also among the places where there may be growing support for decriminalizing sex work, as Nevada and parts of Europe have done, but anti-trafficking groups and local officials appear focused, for now, on more attainable legislative goals.

Delaware and North Carolina, for example, recently classified massage parlors as health businesses, making them subject to regular inspections and other sanitation and safety requirements.

Lawmakers in Illinois, New Jersey, Texas and a dozen other states are also weighing stricter regulations on the massage industry this year.

In Massachusetts, Healey backs proposed legislation to close a loophole that authorities say has allowed illegal spas to operate as unregulated "bodyworks" operations, despite passage of statewide massage parlor requirements in recent years.

At the city and county level, codes limiting operating hours for massage parlors or banning things like buzzer-controlled front doors and back-door entrances have been used in recent years to shutter hundreds of storefronts in San Francisco, San Jose and other parts of California. But officials acknowledge these local measures often just push the industry into neighboring communities without those requirements.

Federal and state prosecutors, meanwhile, have gone after the johns who post detailed, Yelp-style reviews about their massage parlor experiences on online message boards.

In the Seattle-area, for example, authorities shut down a locally run site called The Review Board and charged 35 people, including reviewers and the massage parlor operators on prostitution-related charges in 2016.

Larger massage parlor boards, however, continue to operate.

The Department of Justice did not comment on why that's the case, but the agency highlighted three recent cases in which federal prosecutors have shut down prostitution-related websites and brought charges against their owners, including last year's takedown of the notorious escort listing website Backpage.com.

Experts say part of the challenge is that sites like Rubmaps, one of the most popular national ones, are registered overseas and post disclaimers attempting to distance themselves from their users' posts.

For former massage parlor prostitute Jasmine Grace Marino, the solution is simple: End the demand for paid sex.

The 38-year-old New Hampshire resident says she was forced to work at sites in Connecticut and Maine for five years in her 20s by her then-boyfriend. She's since written a book about her experience and runs Bags of Hope, a Boston-based ministry that helps women who have been trafficked or are dealing with addiction or homelessness.

"Men need to have these conversations," Marino said.