STUART, Fla. – As families shopped around them, a steady stream of men wandered in and out of the Bridge Day Spa, a massage parlor in a strip mall anchored by a Publix Supermarket and a Sherwin-Williams Paint Store. Police say the men engaged in illicit sexual activity with Chinese masseuses in private massage rooms inside the spa, with two or three women reportedly exchanging sexual acts with up to 10 men a day.

Eleven miles away at the Martin County Sheriff's Office, detectives huddled inside a conference room turned high-tech surveillance hub and followed the activity on color flat-screen monitors. Often, they radioed a team perched outside the spa, who would follow the unsuspecting johns and try to identify them, gathering IDs that would number in the hundreds.

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That complex and painstaking – and, to some, controversial – teamwork was at the center of a four-county, seven-month sex trafficking investigation of massage parlors that included hidden cameras, billionaire johns, semen-stained napkins and a $20 million suspected network that stretched from China to New York to Florida.

The investigation, which ensnared nearly 300 suspected johns, including New England Patriots owner Robert Kraft, has sparked a national conversation about human trafficking and renewed calls to strengthen anti-trafficking laws. Police say some of the female spa employees were locked inside the parlors for weeks at a time and made to engage in sexual acts with clients – some as many as 16 times a day.

Overall, hundreds of work hours and more than $400,000 worth of detective work went into the effort police hope will bring down the suspected underground network – and could be replicated in counties across the USA.

“This was a lot more widespread than any of us thought,” Martin County Sheriff William Snyder said. “I don’t think most police agencies or sheriffs know how widespread this is.”

More than 10 people connected to the spas have been charged with offenses ranging from racketeering and money laundering to profiting from prostitution. Only one woman, Lanyun Ma, 49, of Orlando, who ran the East Spa in downtown Vero Beach, has been accused by police of human trafficking, but prosecutors have not formally filed that charge and it’s unclear whether it will proceed.

Through a spokesman, Kraft, 77, who police say visited an illicit massage parlor in Jupiter in January, has denied engaging in any illegal activity. His attorney said Thursday that Kraft will not attend a court arraignment set for March 28, despite a court notice requiring him to appear in person.

Interviews and court documents show the investigation stretched across four Florida counties – Orange, Indian River, Martin and Palm Beach – and netted more than $2 million in seized assets. They also reveal the complexities and challenges of investigating sex trafficking rings, where victims and suspects are often one and the same.

Paul Petruzzi, a Miami-based attorney representing one of the arrested spa managers, said some of the police tactics – such as secretly installing surveillance cameras in private massage rooms – could face legal scrutiny later.

“It’s a very rare and unusual law enforcement tactic to be used,” he said, "and very rare for courts to authorize such a tactic.”

The investigation began on July 6 with a phone call to the Martin County Sheriff’s Office from Karen Herzog, a Florida Department of Health inspector. On a routine inspection of the Bridge Day Spa in Stuart, she noticed suitcases, slept-in massage tables and provocatively dressed masseuses in the strip mall parlor, according to court documents.

Working on Herzog’s tip, Snyder deployed lead detective Michael Felton to look into the spa. For more than two weeks, Felton observed a steady stream of customers, most of them male coming in and out of the parlor, questioned some johns leaving the spa and recovered physical evidence, such as semen-stained napkins from outside trash bins, according to Snyder and court documents.

Felton reported his findings to Snyder and top commanders in the department's Criminal Investigations Division: There was prostitution and likely human trafficking occurring at the spa, he told them. Snyder said he then made a decision: Instead of raiding and shutting down the spa, as most law enforcement agencies would do given such evidence, he would launch a protracted investigation to try to root out any organized criminal rings operating there.

“We would actually see how far we could go in making a case for human trafficking or racketeering,” said Snyder, a former Republican state lawmaker who co-wrote one of the state’s human trafficking laws. “My sense was: These women don’t do this on their own.”

The department assigned up to 10 detectives to the case. They soon noticed that the women, who were all Asian, were often shuttled in expensive cars to other spas: the Cove Day and Florida Therapy spas in Stuart and the Orchids of Asia Day Spa in Jupiter, 17 miles south. Some would enter the spas and not emerge for weeks, he said. Others were driven north to spas in Orange County.

Snyder called the Jupiter Police Department. “I told them, ‘You got a racketeering case going on in your massage parlor,” he said. Police there jumped on the case, mirroring many of the tactics Martin County Sheriff detectives were using. Their focus: the Orchids of Asia Day Spa, a storefront spa in a strip mall in northeast Jupiter featuring a Publix supermarket and several pizzerias.

Snyder also sought help from Homeland Security Investigations, which provided Mandarin interpreters, money and other resources, he said. HSI agents began showing up regularly at the Martin County Sheriff’s Office.

Anthony Salisbury, special agent in charge of the Miami office of HSI, which helped in the case, said one of the challenges in expanding a case from prostitution to sex trafficking is getting the female employees to cooperate. Many suspects in cases he oversees who are alleged sex traffickers end up being charged with prostitution or money laundering instead, he said.

Even more challenging are cases involving Asian women, who tend to have a bigger language barrier and deeper distrust of law enforcement, Salisbury said.

“That is one of the communities that seems to be reluctant to come forward,” he said.

In September, Martin County Sheriff detectives obtained court approval – known as a “break-order warrant" – to install surveillance cameras inside area spas, Snyder said. Officials converted a conference room in the department's headquarters into a high-tech surveillance hub. Four flat-screen monitors showed the inner workings of the spas, in color.

Three detectives – one of which was always a female officer – constantly monitored the screens during the spas’ business hours, from 9 a.m. to about 11 p.m., he said. They clicked off the monitors if a female client entered the massage rooms, focusing solely on male clients, who are more likely to engage in prostitution, Snyder said.

After an illicit act, the detectives would radio an undercover team perched outside the parlor and describe the male suspect as he left the spa. The undercover team would then follow and try to identify the unsuspecting john.

The detectives weren't able to collar every suspected john, Snyder said. Some slipped away while the pursuit team was busy with another client. For every one suspect they identified, another five got away, he said.

"There’s hundreds of men in this county that go to massage parlors where sex trafficking – or at least prostitution – goes on," Snyder said.

Meanwhile, investigators pored over bank and property records of the spa owners, untangling a web of ownership and money that stretched to China. More than $20 million was flowing between China and the Florida spas, Snyder said. The case was growing.

As police in Martin and Palm Beach counties gathered evidence in their case, Vero Beach Police were sending undercover agents into the East Spa in downtown Vero Beach in a separate – and coincidentally concurrent – investigation.

The Vero Beach query began in August after several tips flowed into the department, including an anonymous letter mailed to Chief David Currey detailing how men were streaming in and out of the East Spa, Currey said.

As in the Martin County investigation – and unbeknownst to detectives there – Currey sent undercover agents to monitor the spa, got a break-order warrant to install surveillance cameras inside and set up a room in the Vero Police Department to monitor activity inside massage rooms.

As women were tracked to other nearby spas, detectives from neighboring Sebastian Police Department and the Indian River County Sheriff's Office opened their own investigations, Currey said.

For six months, Vero Beach Police dedicated two investigators, five general crime officers, two supervisors and other personnel to the case, racking up more than $100,000 worth of detective work, Currey said.

"I've been here almost 30 years, and we haven't had an investigation like this in our city in our memory," he said.

Vero Beach Police Detective Sgt. Phil Huddy would later enter one of the Vero Beach massage parlors. There were beds constructed from 2-by-4 planks and mattresses thrown atop, a refrigerator stuffed with food, a break room with a microwave where meals were prepared, and a makeshift shower or spigot coming out of a wall where the women apparently took showers.

“That’s the conditions these ladies were living in,” Huddy said.

Investigators in Martin and Indian River counties learned they were working on similar sex trafficking cases through county prosecutors on the cases, Currey said. They began coordinating efforts.

By February, investigators were ready to move in. On Feb. 19, they launched coordinated raids on the spas and held news conferences announcing the findings.

A major challenge remains getting some of the arrested women to cooperate with investigators.

That challenge came into sharp focus in the wake of the arrests. Snyder watched as one of the women, Lixia Zhu, 48, dissolved into sobs as she told detectives how she came from China to work at a nail salon in Chicago then was forced into sex trafficking. Her passports were locked up and her relatives in China were threatened, Snyder said.

Then, midway through the interview, a Mandarin-speaking attorney from New York showed up. He spoke to Zhu, who immediately stopped cooperating.

“It threw a chill over the entire investigative division,” Snyder said.

Still, there are signs of hope. One woman recounted how she has been shuttled to seven or eight other U.S cities to perform similar acts in massage parlors, showing the reach of the suspected ring, Snyder said. Vero Beach police said they have one cooperating witness who can help prosecutors present a trafficking case.

About a week after the arrests, Martin County Sheriff deputies also received some encouraging intel from the Palm Beach County Sheriff’s Office: U-Haul trucks had been backed up to two massage parlors in their jurisdiction. They were packing up and leaving town.

Snyder said he hopes other law enforcement officials take note and replicate what he has started on the Treasure Coast.

“We found a way to do this,” he said. “If I had my way, we’d bring this methodology to a massage parlor near you.”

Need help? See something?

The National Human Trafficking Hotline is confidential, toll-free and available 24/7 in more than 200 languages.

Call: 1-888-373-7888

Text: “BeFree” (233733)

Chat: humantraffickinghotline.org

Contributing: TCPalm reporters Melissa Holsman, Will Greenlee and Mary Helen Moore.

Follow Jervis on Twitter: @MrRJervis.