From a letter, acquired by ABC News, from John Brennan, White House Homeland Security adviser, to Nancy Pelosi, Speaker of the House:
We believe that significant improvements to the detainee review process have contributed to significant improvements in the results. According to the most recent report to Congress pursuant to section 319 of the Supplemental Appropriations Act of 2009, the Intelligence Community assesses that 20 percent of detainees transferred from Guantanamo are confirmed or suspected of recidivist activity. This figure includes 9.6 percent of detainees who are confirmed recidivists and 10.4 percent of detainees who the Intelligence Community suspects, but is not certain, may have engaged in recidivist activities. I want to underscore the fact that all of these cases relate to detainees releases during the previous Administration and under the prior detainee review process. The report indicates no confirmed or suspected recidivists among detainees transferred during this Administration, although we recognize the ongoing risk that detainees could engage in such activity.
A time line of Guantanamo releases can be found here, courtesy of the Washington Post.