One of the reasons the memo cites for keeping him at the Guantanamo prison is that he "never expressed any genuine remorse for the killing of that soldier." The U.S. said all along that he had confessed to the killing early in his detention, but Khadr's defence lawyers maintained that any confession was extracted, like some other prisoners' admissions, through torture.
http://www.cbc.ca/news/world/story/2011/04/25/us-guantanamo-khadr-wikileaks.html
