The Rockville spa looked like other strip mall salons: plate glass windows, a tiny red awning, posters of smiling women receiving massages.

But a four-month undercover police investigation found the business, known as Rose’s Spa, served as the hub of a sex trafficking scheme involving Chinese immigrants brought to Montgomery County from Flushing, N.Y. The workers were kept in an apartment apparently bought to house them, according to police.

Police said in court documents that the mastermind was former spa owner Emily Zhang Lawrence, 47, who allegedly shuttled the women between the apartment and the salon. Lawrence was charged with five criminal counts, including prostitution, human trafficking and benefiting financially from human trafficking.

“The spa is posing as a normal business but acting as a ‘house of prostitution,’” police concluded in the documents. “Lawrence collects money for those illicit acts.”

The police probe hit a wrinkle when the Montgomery County State’s Attorney’s Office discovered a part-time contract employee “was involved in the case,” according to spokesman Ramon Korionoff, although he would not specify how. At the office’s request, the Frederick County State’s Attorney’s Office took over the investigation to avoid a “conflict of interest,” Korionoff said.

Frederick County spokesman Will Cockey said the investigation is ongoing and that “Frederick prosecutors will be working with Montgomery County Police as it moves forward.”

Lawrence was released from jail on a $7,500 bond in late July, according to court documents. Her attorney, David Martella, didn’t return a call seeking comment. WJLA-TV first reported on the case.

The investigation into Rose’s Spa began in late February when officers received a “web tip complaint” about the salon, according to charging documents.

Two days later, detectives began surveilling the spa. They noticed a pattern: Lawrence pulled up almost every morning in a maroon Honda Odyssey minivan with six women, police said. At the end of the day, Lawrence and the women climbed back into the vehicle and left.

Police determined Lawrence was the owner of the spa and the vehicle, as well as at least two homes in Maryland. Investigators said she was using one of the residences, a condo on Gunners Branch Road, as “a harboring apartment for the Asian female workers.”

Between early March and mid-July, detectives said they stopped about 60 men leaving the spa, all of whom had remained inside for at least an hour. Eighteen men admitted they paid for and received “some sort of illicit sex act,” according to court documents.