President Bush on Wednesday acknowledged the existence of previously secret CIA prisons around the world and said 14 high-value terrorism suspects — including the mastermind of the Sept. 11 attacks — have been transferred from the system to Guantanamo Bay for trials.

He said the “small number” of detainees that have been kept in CIA custody include people responsible for the bombing of the destroyer Cole in 2000 in Yemen and the 1998 attacks on U.S. embassies in Kenya and Tanzania, in addition to the 2001 attacks.

“The most important source of information on where the terrorists are hiding and what they are planning is the terrorists themselves,” Bush said in a White House speech with families of those killed in the Sept. 11, 2001, attacks making up part of the audience. “It has been necessary to move these individuals to an environment where they can be held in secret, questioned by experts and, when appropriate, prosecuted for terrorist acts.”

The announcement from Bush is the first time the administration has acknowledged the existence of CIA prisons, which have been a source of friction between Washington and some allies in Europe. The administration has come under criticism for its treatment of terrorism detainees. European Union lawmakers said the CIA was conducting clandestine flights in Europe to take terror suspects to countries where they could face torture.

Bush said the CIA program has involved such suspected terrorists as Khalid Sheik Mohammed, believed to be the No. 3 al-Qaida leader before he was captured in Pakistan in 2003; Ramzi Binalshibh, an alleged would-be Sept. 11, 2001, hijacker; Abu Zubaydah, who was believed to be a link between Osama bin Laden and many al-Qaida cells before he was also captured in Pakistan, in March 2002.

The list also includes Riduan Isamuddin, known additionally as Hambali, who was suspected of being Jemaah Islamiyah’s main link to al-Qaida and the mastermind of a string of deadly bomb attacks in Indonesia until his 2003 arrest in Thailand.

Defending the program, the president said the questioning of these detainees has provided critical intelligence information about terrorist activities that have enabled officials to prevent attacks not only in the United States, but Europe and other countries. He said the program has been reviewed by administration lawyers and been the subject of strict oversight from within the CIA.

Bush would not detail the type of interrogation techniques that are used through the program, saying they are tough but do not constitute torture.

“This program has helped us to take potential mass murderers off the streets before they have a chance to kill,” the president said. “It is invaluable to America and our allies.”