The Pentagon's top detainee affairs policy appointee has quit the Defense Department just seven months into the job, a Pentagon spokesman said Tuesday.

Phillip Carter, a former Army captain and Iraq War veteran, had been an outspoken critic of Bush-era war on terror detention policy as an attorney and blogging commentator.

He got the job of U.S. deputy assistant secretary of defense for detainee affairs in April, months after President Barack Obama pledged to empty the detention center at Guantánamo. He quit without explanation just days after Obama confirmed in aninterview with Fox News in Beijing that his administration would miss its Jan. 22 Guantánamo closure deadline.

The development apparently took the Department of Defense by surprise. Pentagon spokesman Bryan Whitman declined to say precisely when Carter submitted the resignation, or where he last traveled in a job that took him frequently to Afghanistan, Iraq and the U.S. Navy base in southeast Cuba.

``He has submitted his resignation,'' Whitman said, brushing aside a question of whether Carter was still reporting to work or had already spent his last day on the job. ``A replacement has not yet been named.''

A Defense Department source said Carter, an attorney with a 2004 law degree from UCLA, had already stopped reporting to work by the time the Pentagon confirmed his resignation -- and that his last field trip was not abroad but to Thomson, Ill.

The Obama administration has been considering buying a prison in Thomson to house Guantánamo detainees it would like to move to U.S. soil as part of the plan to close the prison camps in southeast Cuba.

It was not known whether Carter's service in uniform as a civil affairs officer in Iraq helped him fit in to the Defense Department culture where pockets of senior leadership were holdovers from the Bush administration.