Peruvian business student Stephany Flores, 21, was slain in van der Sloot's Lima hotel room May 30. He confessed to the crime but now says he was coerced and tricked and wants the confession tossed.
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Van der Sloot, 21, is also still the chief suspect in the 2005 disappearance in Aruba of Alabama teen Natalee Holloway.
While the lab report shows that the drugs were in Flores' system, observes CBS News Correspondent Elaine Quijano, it's unclear whether Flores took them on her own - or unknowingly.
She says authorities aren't getting any answers from van der Sloot, who stonewalled a Peruvian judge who sought to question him in prison, but the Dutchman continues to talk to the Dutch newspaper de Telegraaf.
He's claiming the FBI set him up, luring him to Peru.
Van der Sloot told the paper his trip to Peru was part of an FBI sting, and that a fellow poker player was in on the operation. He met Flores for the first time at the casino where he was playing.
He says the poker player "arranged and paid everything for me. … Looking back, I can't believe I let myself be lured. I hardly knew that guy. It was just a setup."
The FBI is accusing van der Sloot of trying to take money from Holloway's family in exchange for revealing the location of her remains.
An FBI spokesman says, "This extortion case is a pending legal matter and as such, we simply cannot make any comment on this report."
Legal experts following the case say van der Sloot's credibility is shot. "He has called himself a compulsive liar," remarked CBS News legal analyst Lisa Bloom. "He seems to enjoy talking to the media. He enjoys talking to authorities. So, he may have the right to remain silent, but not the ability."