A Danger sign seen on the front door of the Orchids of Asia Day Spa last week in Jupiter, Fla., in connection to New England Patriots owner Robert Kraft being charged with allegedly soliciting for sex. (Photo by Joe Raedle/Getty Images)

While the accusation New England Patriots owner Robert Kraft solicited a prostitute at a "spa" in Jupiter, Fla., generated news headlines, it also raised awareness of the deeper-buried issue involving the prevalence of sex and human trafficking at ostensibly legal massage parlors across the country.

“Massage parlor networks are a primary place where human trafficking is happening ... The U.S. is replete with them. You can go to almost any city and find one – or many,” said Amy Roth Sandrolini, a spokesperson for The Exodus Road, which fights global sex trafficking.

Illicit massage businesses (IMBs), as they are referred to by law enforcement, have long been known to use beauty and spa messaging as a front for their thriving sex businesses. But what is of increasing concern now is how the IMBs serve as a dark and sinister hub for sex and human trafficking.

A detailed analysis by the anti-slavery non-profit group Polaris in 2017 found more than 9,000 illegal massage businesses up and running nationwide - “hiding in plain sight in strip malls, dotting the sides of highways, and on busy commercial strips in every state.” And while most of the businesses appear rather modest-looking they also made big money - at least $2.5 billion per year, according to the report.

The study found women trafficked into the sex industry arena were most commonly new arrivals from China or South Korea, are mothers with considerable debt or financial pressures, and speak little or no English. The women are often lured into the business by being told “all massage is sexual in the United States,” or that “police in the United States are corrupt” and will only aid the traffickers, not them.

Traffickers are also known to seize passports and take or withhold money from the women, rendering the victims helpless. The women are then threatened and charged “fees” for such things as rent and “training,” making them vulnerable and indebted to their trafficker.

Neither Kraft nor any of those named in last week's Florida bust – on a list that includes a Wall Street billionaire – have been charged with any trafficking offenses. And Kraft categorically denies the existing charges of solicitation. But whatever his fate, investigators seem focused on casting a much wider net, which will expose the degree to which both prostitution and trafficking exist in the region.

Brandon Bouchard, director of media relations for Polaris, said that as law enforcement continues to conduct raids and stings targeting sex trafficking operations, “it’s critical that they do so in a victim-and trauma informed way that doesn’t criminalize the people forced to provide commercial sex services.” That's why many of the women involved in the arrests are considered to be victims, according to law enforcement.

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The IMBs are also “advertised” on an array of public message boards and websites online. Such websites typically require only the checkoff of a box that indicates the visitor is 18 or older - along with a disclaimer the site has "zero tolerance" for underage prostitution.

But once inside, the websites offer troves of “reviews” of various spas and parlors, including explicit information on who provides what and where – along with ratings on the physical appearance of the “masseuse.”

But beyond the disturbing and lucrative nature of individual IMBs, there is often an even wider and more malevolent criminal enterprise at play. According to the Polaris study, “the average IMB connects to at least one other IMB, as well as non-massage venues such as nail salons, restaurants, grocery stores, and dry cleaners.”

“There are networks that run the illicit massage parlor business; like there are large criminal networks that run narcotics,” Sandrolini said.

These connections are utilized to move women from states that are ports of entry, such as New York and California, to IMBs around the country. Overwhelmingly, these connected businesses are used to launder money earned from the IMB,” said the Polaris report. “Networks generally include shell companies that obscure identities of the real trafficking profiteers.”

“Florida’s trafficking bust in massage parlors is just the tip of the iceberg. This is happening all across the country,” said Kevin Malone, president and co-founder of the U.S. Institute Against Human Trafficking, and former general manager of the Los Angeles Dodgers. Noting that many of the IMBs are managed by Chinese organized crime syndicates, Malone said the women involved "are taken usually to New York, where they are trained what to do. They are then put on a circuit of illicit massage businesses, many times not spending more than a few weeks in any given location. They reside in the massage business itself, until they are moved to a new location.”

Concerns have been raised that local law enforcement does not often have the resources required to dismantle such deep and nuanced networks, allowing such exploitation to thrive.

“Several things could trigger an investigation," Sandrolini said. "There could be an assumption of prostitution, money laundering, immigration issues or flags, or a tip from a citizen or potential victim. We know that there is a portion of tips that lead to prostitution, and then there are tips that overlap with human trafficking.”

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Thus most shakedowns rely on multi-agency investigations, which can take months. The mass Florida probe targeted parlors from Palm Beach to Orlando, with almost 200 arrests made, following a six-month investigation into possible human trafficking.

“Massage parlors sit undercover, and it's sad that ’word’ has to get out that guys can come to get a 'happy ending’ before that particular parlor is now on the radar for law enforcement to look into,” said one Florida-based sex trafficking survivor, who now works closely with law enforcement to aid other victims.

In this Oct. 27, 2015 photo, Dawn Stenberg, from the Junior League of Sioux Falls, stands near the group's anti-human trafficking billboard in Sioux Falls, S.D. (AP)

Outside of Florida, charges in states from Wisconsin, Texas, Massachusetts, West Virginia and California have been filed in recent months against spa owners alleged to be running prostitution enterprises. Typically, like in the Florida case, police begin their investigations by scouring online reviews, then conducting video surveillance. Law enforcement will then often conduct routine traffic stops on those leaving to obtain more information.

Nonetheless, experts have established a number of key signs that could potentially point to nefarious sex and labor trafficking pursuits taking place behind the drawn curtain.

“These trafficking rings typically work by setting up a business storefront in any area. It could be residential, urban or suburban,” added Helen Ricci, a patrol sergeant at the Georgetown University Law Center. “There are common features to such establishments; signs indicating massage services for much lower than typical area market rates, the marked presence of a video surveillance cameras at entry and exit ways, locked doors requiring a buzzer, covered windows and doors that limit visibility from the street or to passersby.”

Detective Chris Boughey, of Peoria, Ariz., told Fox News that law enforcement is often alerted to these situations by adjoining businesses.

“Most of these parlors are situated in strip malls. Some adjoining business owners may see increased traffic in and out of these establishments and contact law enforcement. Sometimes, law enforcement receives anonymous tips of the happenings at these establishments,” he said. “We had one case here that the wife of one of these men called police after her husband returned home. Feeling guilty, the husband told his wife what had happened. She was none too happy and called the police.”

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Boughey said the investigative process is a challenging one. The language barrier is often problematic especially in rural areas, when it comes to conducting investigations of suspect locations. Nonetheless, they utilize what resources they do have – from sending undercover officers into establishments and utilizing recording devices – to collect much evidence as possible.

“These cases are hard to prosecute at the establishment level. The johns are easy, but it is a misdemeanor offense. They are usually given a citation and released. A task force approach is the best way to deal with this situation, as these establishments cross jurisdictional lines and following the money takes time, and building the trust of these women is difficult,” he continued. “Most departments and cities don’t have the resources to work these incidents like they should. Additionally, there are few social service agencies equipped to handle, house, etc. these women. The bottom line is that these women are the victims. They are isolated, threatened, and have no resources or people to turn to.”