Reporter Claims to Crack Natalee Holloway Case

— -- A Dutch journalist claims that through his private investigation he has solved the mysterious disappearance of Natalee Holloway and he said today that he knows the American teenager is dead.

Peter R. de Vries said he used a sophisticated network of hidden cameras in what he called an "undercover operation" to obtain evidence that he says "has solved the mystery" of Holloway's disappearance on May 30, 2005.

ABC News has obtained exclusive U.S. rights to this stunning new information caught on tape and will air a 90-minute special edition of "20/20: The Final Hours of Natalee Holloway" on Monday at 9:30 p.m. ET

Based on de Vries evidence, 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.

De Vries said he will reveal his findings in a special program on Dutch TV Sunday.

"Most of the questions, most important questions will be answered in my show. No doubt about that," he said.

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 said he showed some of the key pieces of evidence to the Office of the Public Prosecutor of Aruba on Jan. 24. On Thursday, the island's chief prosecutor, Hans Mos, announced that 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 several months.

The only piece of Sunday's program the de Vries would release was 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 asks Mos. The prosecutor is shown answering, "Now at least the truth is surfacing, and with that we can at least wrap up this case."

The Alabama teenager 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 Joran van der Sloot, who is from Holland, 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.

After his latest release, Van der Sloot did a television interview with de Vries earlier this month in which the reporter challenged Van der Sloot's honesty. Van der Sloot was calm during the interview, but after the cameras were turned off he threw a glass of wine in de Vries' face.

"I was annoying him. I had questions and I was asking for answers, and he wasn't able to give me answers," de Vries said.

"He doesn't have complete control over his behavior," de Vries said of van der Sloot.