About a dozen women arrested over the weekend in a prostitution sting at Honolulu massage parlors won’t be charged with prostitution. Instead, they face the more severe charge of sex assault.

If convicted, the women would have to register as sex offenders and could spend up to a year in jail, while a prostitution charge carries just 30 days.

The new tactic from the Honolulu Police Department is extremely unusual for a law enforcement agency, said legal experts and advocates for prostitutes.

"As advocates, we’re really appalled by what law enforcement has been doing in using this unprecedented approach," said Kathryn Xian, executive director of Pacific Alliance to Stop Slavery. "Why didn’t they arrest the johns or the owners of the property?"

Honolulu police spokeswoman Michelle Yu said the police operation was prompted by public complaints.

"HPD worked with the FBI and Homeland Security Investigations to conduct an undercover operation in response to numerous complaints of prostitution activity and unlicensed cosmetology and massage businesses," she said in a statement. "The operation resulted in more than a dozen arrests as well as several citations. Details of the cases will come out in court."

Hawaii has a strange history with prostitution investigations. Until a year ago, police officers were legally allowed to have sex with prostitutes as part of investigations, an unusual policy that state lawmakers changed last year after The Associated Press highlighted the law. The state is also the last in the nation without a law against sex trafficking, which the Legislature is trying to fix this year.

The standard way police investigate prostitution is to engage in a conversation to agree on a sex act and a price, said Kenneth Franzblau, former anti-trafficking director with the New York State Division of Criminal Justice Services. If that doesn’t happen, there’s no arrest.

Prostitutes know not to have that conversation, and now that prostitutes in Hawaii know police can’t have sex with them, police are trying to get around that difficulty, said an attorney representing some of the women, Myles Breiner. In at least one instance at a massage parlor called Orchid Relaxtion, Breiner said, an officer disrobed, took the woman’s hand and put it on his genitals.

A woman who answered the door at Orchid Relaxation on Wednesday declined to comment. At neighboring China Doll Spa, where some arrests took place, there was no answer.

The sex assault charges don’t require the proof of a plan for money to be exchanged — just evidence the woman touched someone’s genitals without consent.

Bill Johnson, executive director of the National Association of Police Organizations in Washington, D.C., said while the tactic of charging prostitutes with sex assault is unusual, it could be Honolulu’s way of adapting to sex workers’ knowledge of how undercover officers make arrests.

"In fairness to the Police Department, it may be the type of thing where … the alleged criminals modify their activity. So it may be the type of thing where an alleged prostitute may know, ‘If I talk about sex for money I’m going to get arrested,’" he said.

Yu said Wednesday that even before the law was changed, officers weren’t having sex with prostitutes. Police last year asked lawmakers to retain the exemption as a way to keep secret the methods of undercover officers but assured lawmakers that officers do not abuse the protection and that strict internal rules prevent misconduct.

For many of the women, arrest logs list their home addresses as the same as the businesses where they were arrested, which is an indication the women were trafficked, Xian said. A sex assault charge "completely disrupts the trust we’ve fostered over the last several years with law enforcement," Xian said. "It also sends the message to the victims that they can never ever look to law enforcement for help."

Sex trafficking is a very serious issue, Yu said, and Honolulu police will work with federal authorities to "identify victims and get them the services that they need."