DUBLIN — A seven-month undercover police and FBI investigation came to a head Friday as authorities converged on multiple locations across Alameda and San Mateo counties, busting suspected johns, detaining women believed to be sex workers, and ultimately arresting two alleged leaders of a sophisticated sex-trafficking ring.

Investigators say the duo were running the operation like a business, renting out apartment rooms in six locations throughout the Bay Area, using social media to advertise at least a dozen sex workers, and even handing out business cards to johns. Two locations had “receptionists” who welcomed visitors and oversaw operations.

One apartment unit in Dublin contained a lobby area with a large HD television, a dimly lit bedroom with a mattress and incense, as well as two suitcases containing lingerie, bikinis, and other clothes.

Curtis Ng, 45, arrested in Oakland, and Shelly Yu, 49, arrested in Hayward, were charged with pimping, pandering, and conspiracy. But that list of charges may grow as authorities continue to investigate the case.

Authorities say Ng and Yu bought condoms and personal lubricant in bulk and distributed them to the various residences under their control.

The investigation included Dublin police, Alameda County District Attorney investigators, the FBI, and the Alameda County Sheriff’s Office, as well as multiple undercover details.

Several officers on the scene of the suspected brothels in Dublin said this was the biggest sex-trafficking investigation in recent memory.

“Normally, we have massage parlors pop up now and then, but nothing to this extent,” Dublin police Lt. Herb Walters said. “We thought the activity might decrease after the Super Bowl, but no.”

The majority — if not all — of the sex workers reportedly were brought to the United States from China. Five suspected sex workers were detained Friday, but then brought to a victim services unit in Fremont where social workers will try to determine whether any were being held against their will or forced into sex labor. At least one suspected sex worker was overheard telling an FBI agent that she wasn’t in possession of her passport, a common sign of human trafficking.

Authorities began investigating the case after a resident of a San Ramon Road apartment complex complained to police in August that there were signs of prostitution in the area. When detectives took a closer look, they were able to tie Ng and Yu to at least six suspected brothels around the Bay Area, as well as an illegal marijuana grow house in Alameda County, police said.

According to a Dublin police search warrant obtained by this newspaper, undercover officers began posing as customers at suspected brothels in Dublin and San Mateo, and on several occasions paid women for sex services. In each instance, undercover officers terminated the transaction and left, but on at least one occasion a woman there began performing a sex act on an undercover officer, court documents show.

Officers said they also observed Ng and Yu coming and going from various suspected brothels, and at times interacted with them. On one occasion, an undercover officer asked Yu if she had any “bigger girls,” court records show. Yu responded by taking out a cellphone and scrolling through several pictures of other women whom she said would be available for sex, according to court records.

On Friday, police made their coordinated swoop.

After a debriefing at the Dublin police station, in which it was noted that Ng often carries a handgun, police caravans fanned out to six locations.

One caravan of sheriff’s deputies, Dublin officers, an FBI agent and a Mandarin/Cantonese translator discreetly made its way to Connolly Station Apartments in Dublin, a high-end apartment complex within walking distance of BART. They arrived at a residence, barked out orders in English and Cantonese to open the door, and then battered the door open with a ram when they got no response.

Within seconds, the unit’s lone occupant — a woman in her 20s — was in cuffs, and officers were swarming the area, recovering cash, condoms, personal lubricant, cellphones, and other evidence that authorities spent the next several hours processing. But after the scene was cleared, officers took the handcuffs off the woman and told her she was going to be treated like a victim, not a perpetrator.

“I know this seems all crazy and weird right now,” one FBI agent told the young woman, through a Mandarin translator. “But you’re going to be OK; you’ll be taken care of.”

She was later transported to a victim services unit in Fremont.

Ng is being held in lieu of $400,000 bail, and Yu is being held in lieu of $300,000 bail. Authorities are still investigating the possibility that additional victims and suspects are out there.

Contact Dan Lawton at 408-921-8695. Follow him at Twitter.com/dlawton. Contact Nate Gartrell at 925-779-7174. Follow him at Twitter.com/NateGartrell.