While the Bush administration had defended its program of wiretapping without warrants as a vital tool that saved lives, a new government review released Friday said the program's effectiveness in fighting terrorism was unclear. Most intelligence officials interviewed had difficulty citing specific instances" when the National Security Agency's wiretapping program contributed to successes against terrorists, the report said. The program ... played a limited role in the F.B.I.'s overall counterterrorism efforts," the report concluded. The Central Intelligence Agency and other intelligence branches ... could not link it directly to counterterrorism successes, presumably arrests or thwarted plots. The report also hinted at political pressure in preparing the so-called threat assessments that helped form the legal basis for continuing the classified program, whose disclosure in 2005 provoked fierce debate about its legality. The initial authorization of the wiretapping program came after a senior C.I.A. official took a threat evaluation, prepared by analysts who knew nothing of the program, and inserted a paragraph provided by a senior White House official that spoke of the prospect of future attacks against the United States. These threat assessments, which provided the justification for President George W. Bush's reauthorization of the wiretapping program every 45 days, became known among intelligence officials as the "scary memos," the report said. Intelligence analysts involved in the process eventually realized that "if a threat assessment identified a threat against the United States," the wiretapping and related surveillance programs were "likely to be renewed," the report added.