The Bush administration's secret program to transfer suspected terrorists to foreign countries for interrogation has been carried out by the Central Intelligence Agency under broad authority that has allowed it to act without case-by-case approval from the White House or the State or Justice Departments, according to current and former government officials. The unusually expansive authority for the C.I.A. to operate independently was provided by the White House under a still-classified directive signed by President Bush within days of the Sept. 11, 2001, attacks at the World Trade Center and the Pentagon, the officials said. The process, known as rendition, has been central in the government's efforts to disrupt terrorism, but has been bitterly criticized by human rights groups on grounds that the practice has violated the Bush administration's public pledge to provide safeguards against torture. In providing a detailed description of the program, a senior United States official said that it had been aimed only at those suspected of knowing about terrorist operations, and emphasized that the C.I.A. had gone to great lengths to ensure that they were detained under humane conditions and not tortured. The official would not discuss any legal directive under which the agency operated, but said that the "C.I.A. has existing authorities to lawfully conduct these operations."