The Senate Intelligence Committee has released a report on the use of torture by CIA. Numerous cases detailed in that report fall under the definition of war crimes and crimes against humanity, as established by both international customary law and treaties, and law of the United States. At the very least, people performing the torture (regardless of the "I was just following orders" excuse), and people who issued direct orders to torture, should be prosecuted, in accordance with:
US Constitution: "Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted."
UN Declaration for Human Rights: "No one shall be subjected to torture or to cruel, inhuman or degrading treatment or punishment."
United Nations Convention against Torture.