Di Zhang

Di Zhang (Courtesy Baltimore County Police / March 19, 2013)

The directions to the alleged brothel told the men that if they saw a house with green awnings, they'd gone too far. But some of them apparently misunderstood; would-be customers have shown up for years at the nearby house in Towson.

Despite neighbors' complaints, police say, Di Zhang, 42, continued to operate the brothel from a white Colonial-style suburban home on Joppa Road, advertising on websites until this month, when county police and federal agents moved in.

Neighbors said they weren't surprised to learn that Zhang, the operator of Jade Heart Health, had been charged with prostitution and human trafficking. They had long reported the operation to county authorities, and police had raided the business at least five times since 2007, according to charging documents.

Zhang is accused of forcing women to have sex with customers for money.

The business has remained at the house on a strip of boxy midcentury homes along the 1400 block of Joppa Road for years, frustrating regulators and police who say the case highlights the challenges in ferreting out and cracking down on brothels posing as legitimate businesses.

Prostitution and human-trafficking charges against Zhang in 2008 were dropped.

This time, federal Homeland Security agents were involved, raising the possibility of stiffer penalties that could close Zhang's business for good. According to state regulators, she hasn't had a massage license since 2004.

Zhang, who was released a day after the March 8 raid, has not entered a plea to the charges. She did not return telephone messages seeking comment. No one answered the door at her home, an attorney who represented her in previous cases did not respond to several phone messages and online court records do not show a current lawyer listed.

Police and organizations that work with victims say there are several businesses in the region like the one that Zhang is accused of running.

Such establishments often spring up along the York Road corridor, the Baltimore County Commission on Women reported last year. The road runs alongside Interstate 83, providing easy access to New York and Pennsylvania.

But as soon as authorities shut them down, they pop up elsewhere.

"You have to be able to react quickly," said Rod J. Rosenstein, the U.S. attorney for Maryland. And even then, he said, it's a challenge to get the victims to cooperate — they are often distrustful of law enforcement and sometimes unaware they are even victims.

Federal authorities don't generally take up prostitution or human-trafficking cases unless minors or foreigners are involved, or if it appears an operation spans state lines. Prosecutors declined to discuss details of the case.

The investigation began with a complaint from a neighbor, according to charging documents.

Homeland Security Special Agent David Snyder, the supervisor for investigations in the Baltimore region, said cases often start with such reports.

"We get leads all the time from neighbors," he said. "They see men coming out of the massage parlor. They are suspicious.

"The police are also great resources for us. They ride by these places. They get calls for service."

Gretchen Sarkin, president of the Loch Raven Village community association, said she had heard about the house: "Suspects walking back and forth, in and out of the back," she said.

Those who investigate human trafficking said no place is immune to the trade.

"It's pretty shocking to people in Baltimore County that there are lots of these types of spas," said Melissa Snow, the director of TurnAround, a center based in Towson that helps abused and battered women.