Denver- Four years after launching an investigation into a network of massage parlors that authorities said offered sex in back rooms, Denver police are once more cracking down on some of the same locations for the same crimes.

A series of search warrants detail an increasingly sophisticated and persistent operation that has dug itself in and shows little sign of leaving.

“These are very difficult and complex investigations,” said Dave Fisher, Denver police division chief. “It’s always a challenge when you are dealing with a criminal enterprise that is so lucrative.

“But I’m confident we can maintain the upper hand on massage parlors in Denver and we continue to do that.”

In the past month, police have raided massage parlors around the city, each allegedly a front for a Korean brothel.

The spas have names like the Yokohama Spa and VIP Spa. While many of the raided businesses have changed their names from what they were four years ago, some, like VIP, didn’t. Police believe that new owners simply came in and picked up where the previous ones left off, according to the warrants.

“The transfer of ownership greatly complicates these kinds of investigations,” Fisher said.

On Aug. 4, vice detectives raided the VIP Spa, 1850 S. Federal Blvd. Inside they discovered a topless masseuse and a man wearing only a red condom. The man told police he had paid $80 for sex, according to a search warrant.

The woman later told police she was using the money she earned to support a preschool she owned in California and promised to stop if they let her go.

“I know, I stop, I stop, I won’t do it anymore. I will just want to go home, please let me go,” she told police, according to the warrant.

Police also discovered about $3,800 hidden throughout the small house, “female sexual enhancement items” and a large number of unused condoms hidden inside table legs.

Police say the woman had parked a Lexus and a Mercedes in the parking lot of the spa and used them to store things from the business. Detectives discovered a bag filled with hundreds of unused condoms on a back seat of the Lexus. The Mercedes was filled with credit card receipts, advertisements for the spa and bills.

The scene was much the same in 2003, when investigators raided the VIP Spa as part of Operation Rising Sun.

The federal investigation was kicked off in 2001 to look into dozens of massage parlors nationwide that were suspected of being fronts for brothels. The VIP Spa was among 11 in the Denver area on the list.

In August 2003, an undercover officer received a massage at the VIP Spa from a woman who then offered to have sex with him for $100.

Investigators at the time said they believed Korean women who had been smuggled into the country and transported from California to Denver were forced to live at the spa and work as prostitutes.

But some of the women contacted by the Rocky Mountain News denied they were forced into prostitution.

The walls of one spa were concrete, the lights pink, the music piped in through cheap speakers in another room.

A woman who called herself Kim sat curled in a faded loveseat playing with her toes as she talked about her life and job.

She stressed one point – that she is not a prostitute.

But police say she is and federal court documents say the spa where she works is home to a covert sex parlor.

And if it were, she asked, would it really matter?

“It wouldn’t bother me, would it bother you?” she asked, smiling.

Police say that most of the women in the spas raided last month are recruited from Asian countries where they were working as prostitutes. The women are told they can make a lot more money in the U.S. working in massage parlors, according to court records.

Fisher said the police department is commited to driving the illegal adult massage parlors out of the area.