The entire operation began by chance in a Dutch casino. Van der Eem, a bold and emotional Dutch businessman who had never met van der Sloot before but followed the case with a passion, ran into his eventual target and claimed he decided on the spot to try and get inside the head of a man he believed had disgraced their nation.
"Getting involved is hard, you know, but I knew if I could have done something, or can do in the future, I will," he said.
"And when you saw him in the casino,'' ABC News' Chris Cuomo asked him, "you said this is your chance?"
"And I will finish him,'' van der Eem replied, explaining that he'd grown frustrated watching van der Sloot tell one news organization after another that the Aruban people are "with'' him.
"The Aruban people are good people who will give you the trust, because the justice system sometimes fails a little bit. But I knew there was something wrong in this story,'' van der Eem said.
Van der Eem adopted a strategic approach.
He walked up to van der Sloot and said without introduction, "Hey murderer'' in Papiamento, the official language of Aruba.
He said he knew that "nobody on the other table would understand what I said to him because we live here in Holland and … we talk Dutch.''
After getting van der Sloot's attention, van der Eem said, "I went, 'How are you doing? Fine?' And I walked away. I went smoking a cigarette. And two minutes later he came [over].''
Van der Eem struck up a conversation with van der Sloot and began asking him how he spent his summer. The young man replied that he played poker and smoked marijuana, van der Eem said, and the "game was started."
Like a seasoned undercover agent, van der Eem began to identify his target's weak spots and exploit them as best he could.
"Playing poker, OK, that's one addiction. And smoking weed every day, that's-- OK, I'm going to hit on that one,'' Van der Eem said.
"I asked him how much he paid for a gram of weed. He told me 70 Euro. I told him, 'Are you crazy? I'll give you my number. And I'll help you with 100 grams [for] 300 Euro.' He told me, 'Yeah, man, that's good.' It took exactly two days for him to call. And as soon as he made that call, I knew what I was going to do with him."
Van der Eem said that "the game was started with saying, 'matung.'
"And then it proceeds.''
Van der Eem said he spent nearly every day for two months with van der Sloot. The pair smoked marijuana, drove around Holland and played poker late into the night, van der Eem said.
The next step, he said, was holding van der Sloot's interest. He suggested that they monetize their common interest in marijuana by growing and selling pot in the Netherlands.
"We got to start something up, '' van der Eem said he told van der Sloot. "Let's go green!
"He loved it. Oh, he was enjoying it."
So, after months of priming his target, he said he went to de Vries.
"He came to me and said, 'Joran is trusting me completely,'' de Vries told ABC News. 'And I think he knows more about [the Holloway disappearance] and he will finally tell me. Are you interested?' And I said, 'Of course I'm interested. Stay as close to him as you can because this can be very important.' And well, that's what he did.''
Van der Eem said he gained the other man's trust slowly, by playing the role of a criminal and repeatedly telling van der Sloot that he didn't care about the case.