It's one of the Pentagon's most sensitive and carefully guarded secrets: Who interrogated the prisoners at Guantánamo?

So it came as a surprise last month when a Pennsylvania congressman seeking reelection campaigned as the only member of the U.S. Congress to have interrogated a Guantánamo detainee.

It didn't work. Rep. Chris Carney, a Blue Dog Democrat, lost his mainly Republican district to a former federal prosecutor.

But the revelation raised eyebrows in Washington, where Carney served on the Homeland Security Committee, as well as questions about the Pentagon's effort to keep the identities of the people who have conducted Guantánamo interrogations secret.

Military commissions judges shield the interrogators' names and call them only by aliases such as Interrogator No. 1.

During recent hearings in advance of Canadian Omar Khadr's guilty plea to war crimes charges, Interrogator No. 3 testified in his dress uniform in court but the military insisted a courtroom sketch artist blur his features.

At the Pentagon, a spokeswoman said Carney, who remains a reservist subject to call up, did not need permission from Military Intelligence circles or others in his chain of command to disclose his interrogator role on his résumé.

``That was his call,'' said Army Maj. Tanya Bradsher. The Department of Defense does not make public the identities of its interrogators, she said, to protect its troops.

Whether the revelation will hurt Carney's chances to be used again as a military interrogator ``will simply have to be taken into account when this reservist is being considered for future assignments,'' she said.

The revelation, however, caught fellow lawmakers by surprise.

Carney, 51, apparently never revealed to his Democratic colleagues the top-secret portion of his intelligence résumé, even as he opposed President Barack Obama's plan to empty the Guantánamo prison camps and move some war-on-terror suspects' trials to civilian courts.

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