Officials declined to provide the exact numbers, saying they were classified.
These new figures, first reported by the Washington Post, represent only a portion of those former detainees who returned to the battlefield: those known to have killed Americans. A 2016 report from the Director of National Intelligence estimates that 17.5 percent of 676 released detainees returned to the battlefield. Nearly half of them are now dead or in custody, the report stated.
Of the at least 12 former detainees suspected of killing Americans, nine are now dead or in foreign custody, officials believe. They also say the killings did not occur recently and that all the former detainees involved were released during the George W. Bush administration.
At its peak, the detention facility at Guantanamo housed more than 800 detainees. The Bush administration ultimately transferred more than 500 out of Guantanamo and 158 have been transferred out under the Obama administration.
There are 80 detainees still at Guantanamo, with 30 of them cleared for transfer to other countries.