Snaking through the streets of Cambodia's capital city, we're on the tail of suspected American pedophile Harvey Johnson. The 57-year-old failed real estate developer from Arizona has been teaching English out of his house in Phnom Penh for two years. His position has given him ample opportunity to get close to children, authorities say.
Cambodia has long been a top destination for pedophiles from the United States and all over the world, according to law enforcement officials and humanitarian groups.
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Unbeknown to Johnson, he has been under surveillance by a local nonprofit group called APLE, which has made it its mission to identify suspected foreign pedophiles and to help gather enough evidence for the police to make an arrest.
Johnson is accused of using his position as an English teacher to molest several underage girls. APLE investigators tell ABC News they first noticed Johnson in October 2007 when they claim he was behaving suspiciously with children in public.
"He was seen touching; he was seen caressing those children. That's why we opened a case against him," APLE agent Samleang Seila tells us.
APLE used an undercover "spy agent" to befriend Johnson. Its hidden cameras caught Johnson allegedly selling child pornography to the agent. The agent recorded hours of conversations in which APLE claimed Johnson talked freely about molesting young girls.
"Slipped my little finger right up her," he told the APLE agent in a hidden audio recording. "But it was snug. She's very small."
After nearly two years of building its case, APLE agents told us that they believed they'd finally provided the police with enough evidence to get an arrest warrant.
In the final days of its investigation, it agreed to give ABC News extraordinary, behind-the-scenes access.
APLE officials took us to Johnson's home, where agent Rattana Rong told us that four female students had allegedly entered Johnson's home.
Investigators, who told us they had been following Johnson night and day, seemed to have surveillance down to a science. There were agents staked out in strategic locations -- some posing as motor bike taxi drivers, another as a vendor of cold drinks -- and two more agents are planted in Johnson's neighbor's house.
To our surprise, we spotted Johnson himself, sitting at an outdoor restaurant, and he struck up a conversation with us.
"How long are you in town?" he asked.
"Just a couple of weeks," I replied.
"Oh, a couple of weeks?"
"You're just enjoying all the sights ... and diversions."
He gives us pointers on where to stay and what to see, mentioning Martinis -- a bar in the city we are told is notorious for being a place where visitors can solicit prostitutes. Johnson gave us his phone number, and we left before he got suspicious.
APLE is now part of a coordinated international crackdown, involving the Cambodian and American police, the FBI and ICE international law enforcement, called "Operation Twisted Traveler."
One of the suspected pedophiles APLE recently helped to bust was Michael Dodd, a registered sex offender in Florida. Dodd was accused of attempting to arrange his marriage with a 14-year-old girl. We found him awaiting trial, along with the girl's mother.