State Department envoy Clifford Sloan announced plans Monday to step down from his role as negotiator of prisoner transfers from the detention center at Guantánamo Bay, Cuba. Though Sloan’s surprise resignation comes as tension grows over the Defense Department’s delayed release of prisoners from the U.S. military prison, Sloan dismissed it as the reason for his departure and said he always intended to serve a maximum of 18 months. Since November, 17 detainees have been transferred and over the weekend the Pentagon announced the transfer of four more, to Afghanistan. Sloan will return to his Washington law practice after his resignation takes effect Dec. 31. A senior official said the White House has not yet decided on a successor, The New York Times reports.