Why the warning, I wonder? His passing will be some sort of signal?
Revenge attacks: the Muslim world is even more given to conspiratorial thinking than are the fever-swamps of the American left (and right). His death in custody will be painted as a murder by nefarious means (e.g. since he coughed up blood, lunatic Muslims might spread the rumor he was intentionally exposed to ebola), and used as the basis for a call for attacks or as motive for already-in-place cells or 'lone wolf' jihadis to launch attacks.