ROSEMEAD, CA — Sgt. Tri Hoang sat at work two weeks ago, surfing the Internet for prostitutes.

The research was done in preparation for an online sex-ad sting last Tuesday and Wednesday that led to 37 arrests, including 30 for prostitution and two for pimping.

The Los Angeles County sheriff’s sergeant said authorities became “desensitized” after scanning thousands of online ads, many with scantily clad women in compromising positions.

“With this operation, we’ve made it public that this problem exists,” Hoang said. “I would say most people are unaware that prostitution is going on in and around their communities.”

Prostitution in the San Gabriel Valley is a widespread and increasing problem, and advertisements span the Internet and are also found in newspapers, authorities said.

Last week’s operation included the Monterey Park, Alhambra, San Gabriel and Pasadena police departments, U.S. Immigration and Customs Enforcement, as well as the South El Monte, Rosemead and Temple City teams of the Sheriff’s Department.

In Rosemead, neighbor complaints sparked a similar but smaller operation in November when seven prostitution suspects were arrested. The success of the sting prompted authorities to develop an operation that would span a larger section of the San Gabriel Valley.

“This is one of the activities that is difficult to address, and it takes a lot of behind-the-scenes workups,” said sheriff’s Lt. Sheila Sanchez.

Mario Estrada, who leads the sheriff’s special vice unit, believes that many of the women are victims of human trafficking.

“What we saw was the face of prostitution,” Hoang said. “What is behind it is enslaving, pimping, money and narcotics.”

Arrests during the two-day sting were at massage parlors, hotel rooms, houses and apartment complexes.

But these types of stings present many dangers, said Estrada, who has worked for the county’s special vice unit for nearly seven years. The unit was also part of the two-day sting.

Men could be at the apartment or condo. They could be the pimp or a “john” buying sex. There could also be other women there, working. There could be weapons, surveillance cameras and lookouts.

Besides this, undercover officers need to time the incident so that they have proof of a violation, but need to stall the sex act.

The decoys are often fully undressed before their backup officers enter the premises.

And during all this, they need to remember to get a verbal violation.

“These girls are not stupid,” Estrada said. “They know cops have to ask them to have sex for money.”

On Tuesday, nearly 50 law enforcement officers, including some working undercover, crammed into the command post at the Rosemead Community Center at 9 a.m.

Each team was given a stack of ads, many from the Craigslist Web site.

While posting solicitations for prostitution is illegal, many ads get around this by offering things such as: “Sexy special massage to give you full release! I’m cute and pretty, with beautiful bra! Give you happy ending!”

Most of the arrests were at brothel-style locations, which are set up similarly throughout Los Angeles County. Lingerie bags, Kleenex boxes, rolled-up towels, condoms and bottles of lotion, Listerine and baby oil were found throughout the raided places.

Each room was set up with a nightstand, a bed – usually only a box mattress on the floor – and a heater.

On the second day, authorities folded massage parlors into the sting.

By targeting prostitutes, authorities find a way to get into a world that is often associated with much more serious crime, Hoang said.

“People consider this a victimless crime, but it’s not. You are hurting people, you’re using people,” Estrada said. “These people are forced to go out there.”

Authorities were also searching for suspects involved in human trafficking. Of the nearly 25 locations that were visited, one woman said she was a victim of human trafficking.

“We were hoping for more,” Estrada said. “A lot of the victims are not going to come forward. They don’t want to get themselves hurt or killed.”

Lori Haley, spokeswomen for U.S. Immigration and Customs Enforcement, said these cases are very difficult and complex.

“In the San Gabriel Valley, we haven’t had anything when anyone recently has been charged with human trafficking,” Haley said.

However, she cannot comment on any pending investigations.

“Everyone seems to realize that it is a widespread problem, but we just don’t have those statistics,” Haley said. “It is probably happening a lot more than we think. Getting the victim to come forward is a problem.”

Last year, a human trafficking case out of Los Angeles led to the conviction of nine people from a sex ring where victims as young as 13 were promised jobs, then suffered threats and beatings as they worked as prostitutes in brothels.

“Prostitution is a misdemeanor,” Hoang said. “However, all the activities that surround prostitution are more of your predominant crimes, such as guns, narcotics, pimping and human trafficking.”

But not all women arrested last week appeared to be forced into it.

Dawn Smith, who was cited on suspicion of prostitution at the Oak Tree Inn in Monrovia, sat in a chair Wednesday as authorities raided the room.

Just minutes before they arrived in the room, a decoy paid her $200 in exchange for sex. As soon as the decoy paid, he was asked to undress. A man Smith later identified as her boyfriend came to the door to collect the money. Minutes later, authorities came through the door and arrested Smith and her boyfriend.

Under questioning, Smith said she did not intend to have sex. She was an entertainer, she said, who was working to pay off a $1,000 cell phone bill that her 15-year-old daughter racked up.

“This is why I quit two years ago,” Smith said, handcuffed. “I’m not going to do this anymore.”

Estrada said that too often he cites women multiple times or will bust the same “massage parlor” more than once.

He knows that some chose this way of life and will fall back into it, and others are forced into it but are afraid to come forward.

“We’re doing as much as we can,” Estrada said. “But if we could get at least one woman to come forward, then we are making a difference.”