But the releases are an inherent gamble: The freed detainees are often notorious fighters who would not be released under the traditional legal system for military prisoners in Afghanistan. They must promise to give up violence — and U.S. officials warn them that if they are caught attacking American troops, they will be detained once again.

There are no absolute guarantees, however, and officials would not say whether those who have been released under the program have later returned to attack U.S. and Afghan forces once again…

Unlike at Guantanamo, releasing prisoners from the Parwan detention center, the only American military prison in Afghanistan, does not require congressional approval and can be done clandestinely. And although official negotiations with top insurgent leaders are seen by many as an endgame for the war, which has claimed nearly 2,000 U.S. lives, the strategic release program has a less ambitious goal: to quell violence in concentrated areas where NATO is unable to ensure security, particularly as troops continue to withdraw. The releases are intended to produce tactical gains but are not considered part of a grand bargain with the Taliban.

U.S. officials would not say how many detainees have been released under the program, though they said such cases are relatively rare.