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He has helped craft policies that will allow hundreds of prisoners held by the U.S. military in Afghanistan to challenge their indefinite detention under a new review system. Carter was also involved in the administration's effort to close the detention facility at Guantanamo Bay, Cuba, which holds 215 terrorism suspects.

His departure comes at a critical moment for the administration, which is trying to find a location in the United States to stage military tribunals and place some of the Guantanamo inmates in indefinite detention. He spoke last week at a community forum in Thomson, Ill., the site of a maximum-security state prison that the administration is seriously considering to house some detainees from Guantanamo.

The administration is trying to resettle or repatriate about 90 detainees, prosecute 40 others in either federal courts or military commissions and possibly hold up to 75 others in some system of prolonged detention under the laws of war.

Carter worked on a Justice Department-led task force, which will offer recommendations to President Obama on future detention policy.

Carter, a critic of detention policy under President George W. Bush, filed friend-of-the-court briefs in Supreme Court cases challenging his administration's policies, including the Hamdan v. Rumsfeld case, in which the court struck down that administration's system of military commissions for trying Guantanamo detainees.

Carter, who worked on Vets for Obama during the presidential campaign last year, attempted to build relationships with the human rights community, which remains critical of the current administration's decision to employ a reformed system of military commissions. Even so, groups such as Human Rights Watch and the American Civil Liberties Union spurned an invitation to visit the Guantanamo Bay facility, saying that such a tour would be meaningless unless they could meet independently with detainees.

But activists praised Carter's efforts.

"Phil was very instrumental at ensuring that human rights organizations were able to bring their concerns to the table and have them discussed," said Jonathan Horowitz, who works on detainee policy as a consultant for the Open Society Institute. "It's a practice that I hope his successor will follow."

(Editor's Note: Mr. Carter formerly wrote for washingtonpost.com's blog Intel Dump. He stopped writing for the now-defunct blog in July 2008 to work for Barack Obama's presidential campaign as its national veterans director.)