When Ryan Cantrell worked as a high school resource officer with the Hayward Police Department, he sometimes had to track missing teenage girls. What he found was disturbing.

"The girls were not necessarily runaways but they were lured into the sex trafficking trade," he said. "They take 13- to 14-year-old girls, they recruit them off the street or in the mall, and the average time is about 48 hours to get them out on the street."

Cantrell, a Pleasanton resident, soon found himself transitioning each workday to the violent sex world, where the exploiters tattooed their victims, some as young as 9 years old.

"There are designated areas known as prostitution tracks, where pimps bring them to work the streets," Cantrell explained. "An example of that is International Boulevard in Oakland. You'll find 15 to 40 girls or women working at any time."

Concerned with the grim situation, he applied to the vice unit, where he learned to recognize human trafficking in many forms.

"There was an Asian brothel working in our city -- that particular case really got me into it," Cantrell recalled. "This lady and guy were running it since 2004, and we started working the case in 2010."

He began to realize that prosecuting human traffickers is also important because it is related to other illegal activities, including property crimes, violent crimes and drug trafficking.

"A lot of drug traffickers have gone from dealing drugs to being pimps and exploiting girls," he said. "Once you sell a pound of cocaine it's gone, but you can put a girl on the track and sell her over and over again.

"The girls fall in love with these guys. It's hard to get them to turn against their pimps."

Cantrell calls the men "exploiters of children and women" rather than "pimps," a term he believes has been glorified on television and the movies.

"They can make over $700,000 a year -- three to five girls working every day, $500 average a day, per girl," he said. "There's big money to be made."

The Asian brothel case was a year-long investigation, Cantrell said; it had 12 different brothels in Alameda, Santa Clara and Contra Costa counties, working out of apartments and next to businesses, and moving frequently.

"We were getting lots of complaints," he said.

"They were complex organizations, fed by Asian organized crime," he said. "It was a year-long investigation, then we found out the girls were actually being trafficked. We recovered eight victims -- all these girls were Taiwanese nationals in the U.S. legally on tourist visas."

The women were recruited in Taiwan, and most had families so their exploiters could threaten to reveal they were working as prostitutes. Ultimately the exploiters were convicted.

"The tough part was keeping the victims here to testify," Cantrell said. "That's key."

He learned so much from this case and others about human trafficking that he decided to write a book that would help other investigators. It came out in January, and he hears back from detectives who use it for interviewing.

"My expertise is human trafficking and brothel investigation and domestic child exploitation," Cantrell said. "Sex trafficking is one component. It's a brutal, horrific crime."

He said most police organizations have guidelines but he wanted to help others recognize the problem as well as pursue prosecution.

"If you have children, especially girls, you need to read this book," he said. "It's in every neighborhood, every back yard, in every city, in Pleasanton."

Cantrell has a bachelor's degree in Criminal Justice Administration from Cal State Hayward and formal training in Child Prostitution Investigations and the Commercial Sexual Exploitation of Children from the Department of Justice.

"We know there are girls that operate out of hotels in Pleasanton," he said. "There are any number of websites. You can order up any color, any size, any fetish, in any location -- there's a list of girls that fit that criteria, and within an hour you have a girl at your house or an appointment to visit one."

The websites operate offshore, out of the reach of American law enforcement, he said; Craigslist was popular for awhile but the website took a stand and eliminated it.

"Men who share chats have their own system of intelligence," he said. "I love catching these guys who think they can avoid the cops. They can be very prominent guys."

Cantrell's vice squad started a task force with others in the area to pool resources and train officers in how to identify sex trafficking.

"There is so much more that can be done," Cantrell said. "My motivation to write the book was to get word out about it. I've had calls from social workers who said it is good information to have."

He also did a training for PG&E engineers. Massage parlors are often a cover for brothels, he said, and if PG&E workers do an inspection they should know to look for illegal living quarters and girls who are cowering or who look really young. Some of these establishments are unlicensed and provide sexual services, he said, while others are quasi-legitimate -- the owner encourages the women to offer services.

"They are very hard to shut down," he said.

Another red flag might be if a customer needs to be buzzed in, or if it doesn't sell gift certificates.

"My focus is share information with other cops to conduct good comprehensive investigations that arrest exploiters and recover victims," Cantrell said. "My priority is recovering the girls and arresting and convicting the exploiters."

Cantrell is now a supervisor in the patrol division. But he continues to spread the word about human trafficking, which includes not just the sex trade, but slave labor and extracting organs to sell.

"More people are in slavery right now than ever have been in the history of the world -- on every continent and in every country," Cantrell said.

"The Detective's Guide -- Modern Slavery: Investigating Human Trafficking" by Ryan D. Cantrell is available online.