(CNN) -- Federal agents have backed up al Qaeda captive Khalid Sheikh Mohammed's confession in the killing of journalist Daniel Pearl by using photographs of the veins in his hands, according to a new report released Thursday. Mohammed confessed to beheading Pearl after his 2003 arrest in Pakistan. But the U.S. admission that he had been subjected to "waterboarding" -- a practice historically treated as torture -- while in CIA custody cast doubt on the reliability of his confession, according to a lengthy investigation of the Pearl case by the Center for Public Integrity. According to the report, the FBI and CIA used stills from the video of Pearl's killing to match the patterns of the veins in Mohammed's hand in 2004 and repeated the process in 2007, after Mohammed repeated his confession during a hearing at the U.S. prison camp at Guantanamo Bay, Cuba.
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Officials used 'vein-matching' to tie Khalid Sheikh Mohammed to Pearl's killing, report says
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Mohammed has said he killed the journalist, but has not been charged
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The report is a result of a three-year investigation
Mohammed and one of his nephews remain in U.S. custody, another man is being held in Pakistan on unrelated charges and another of @placeholder's nephews is believed to be in custody, "but his whereabouts cannot be confirmed."