"That is what he wanted to hear, so I told him what he wanted to hear," Van der Sloot said, adding that he never fully trusted the man to whom he'd described his encounter with Natalee Holloway.
"It is so stupid, it is so stupid, it is really stupid," Van der Sloot said, his voice cracking.
Based on de Vries tapes, the chief prosecutor in Aruba announced Thursday that he is reopening the case.
De Vries also showed his findings to Holloway's mother, Beth Twitty.
"She told me she kind of knew it already that Natalee wasn't alive anymore, but when you get this message it's still, yeah, a kind of relief," de Vries told ABC News.
Twitty admitted as much to ABC News.
"Now, with the knowing," she said, "it lets you put some things to rest. And that finally, finally, finally it's over."
According to the DeVries Dutch TV special, De Vries built his case with the help of van der Eem, 34 an Antillian who spent his youth in Curacao and Aruba but who has lived in the Netherlands for many years.
Van der Eem said he met van der Sloot at a poker table in a casino, and they talked about starting a marijuana-growing business. The two later would talk about and smoke marijuana during lengthy recorded conversations.
But van der Eem said he really wanted to bring van der Sloot down and decided to set a trap. Soon, he approached de Vries, the famous Dutch crime reporter, about working undercover, and de Vries hired him.
"I knew what I was doing," van der Eem said. "I had no emotions for this kid. The mother deserves an honest answer for what happened to his daughter."
The mystery behind the disappearance of the blond teenager has eluded efforts by the Aruba police and even the FBI for more than two years.
De Vries showed some of the key pieces of videotape to the Office of the Public Prosecutor of Aruba Jan. 24. Thursday, the island's chief prosecutor, Hans Mos, announced he has "intensified [the] investigation of Natalee Holloway due to recently received information."
Mos had announced publicly last month that he had closed the investigation after an exhausting and often frustrating two-year probe.
"This information may shed a new light on the mode of which Natalee Holloway has died and the method by which her body disappeared," Mos said.
He credited de Vries with uncovering the leads and said, "This information may help considerably in the solution of the mystery of Natalee's disappearance."
De Vries said that he and his team had been conducting an undercover operation for five months.
Earlier this week de Vries released a conversation between himself and Mos after he showed police the evidence he had collected.
"Am I sitting opposite a happy man now?" de Vries asked Mos. The prosecutor is shown answering, "Now at least the truth is surfacing, and with that we can at least wrap up this case."
Holloway, from Alabama, disappeared after a boozy night in an Aruban nightclub during a high school graduation trip. She was last seen outside Carols 'n Charlies bar with van der Sloot, who is from the Netherlands, and the brothers Deepak and Satish Kalpoe, from Aruba. They have twice been arrested and released because of a lack of evidence. They have all denied any involvement in her disappearance.
In the de Vries TV special, Van der Eem said he rarely pressed van der Sloot on the Holloway case early on -- but van der Sloot occasionally talked about it anyway.