WASHINGTON — The FBI has reopened an inquiry into one of the most intriguing aspects of the pre-Iraq war intelligence fiasco: how the Bush administration came to rely on forged documents linking Iraq to nuclear weapons materials as part of its justification for the invasion. The documents inspired intense U.S. interest in the buildup to the war — and they led the CIA to send a former ambassador to the African nation of Niger to investigate whether Iraq had sought the materials there. The ambassador, Joseph C. Wilson IV, found little evidence to support such a claim, and the documents...