Last week, a jury in Guantanamo Bay convicted Salim Hamdan of a war crime. But according to a new book, the government might have turned down an opportunity to turn Hamdan into a friendly source.

In The Challenge, New York Times Magazine writer Jonathan Mahler reports that one of the FBI’s top agents believed Hamdan was ready to provide key evidence against top level al-Qaida players. The government’s best shot at getting the info, the agent argued, would be to offer Hamdan a plea deal in federal court. Instead, the administration sent Hamdan to the U.S.‘s first war crimes tribunal in 60 years, which ended in a less-than-complete victory for the government.

Mahler reports that FBI agent Ali Soufan—one of the few FBI agents who spoke Arabic—had interrogated Hamdan at Guantanamo Bay and found he had lots of potentially useful information. While Hamdan was only a bit player, he was a valuable witness. For example, Hamdan had apparently witnessed Khalid Sheikh Mohammed debrief Osama bin Laden on the 9/11 operation.

Instead of prosecuting Hamdan, Mahler writes, Soufan thought he “could persuade Hamdan to plead guilty and cooperate with the government in exchange for a lighter sentence.” After all, Soufan argued, if the government moved ahead with a tribunal, Hamdan would eventually get a lawyer and clam up.

The administration, of course, declined to take Soufan’s advice and opted for a tribunal.

“Soufan’s access to Hamdan was immediately cut off,” writes Mahler, “and the FBI lost a crucial source of information, as well as a potential key witness in other al-Qaida trials.”

We’ve excerpted the relevant passage (PDF). (Also, as a bit of a bonus, we’ve posted the book’s prologue.)

We spoke briefly with Mahler about the incident.

“Soufan was Hamdan’s main interrogator and the FBI’s foremost expert on al Qaeda,” Mahler told us. “He knew how valuable Hamdan was as an intelligence asset, particularly if he agreed to testify at other trials as part of a plea deal - which Soufan thought he would do. But he was ignored.”

A spokesman at the FBI declined to comment: “We’re not commenting on any individuals who are going before or who have been to the commission.”

We’ve also tried contacting Soufan. (He was profiled by the New Yorker but otherwise has rarely spoken to the media.)

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