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 the claim, and the documents were later deemed to have been forged.

But President Bush referred to the claim in his 2003 State of the Union address in making the case for the invasion. Bush's speech, Wilson's trip and the role his wife played in sending him have created a political storm that still envelopes the White House.

The documents in question included letters on Niger government letterhead and purported contracts showing sales of uranium to Iraq. They were provided in 2002 to an Italian magazine, which turned them over to the U.S. Embassy in Rome.

The FBI's decision to reopen the investigation reverses the agency's announcement last month that it had finished a two-year inquiry and concluded that the forgeries were part of a moneymaking scheme -- and not an effort to manipulate U.S. foreign policy.

A senior federal law enforcement, who spoke on the condition of anonymity because of the sensitivity of the investigation, said the request to reopen the inquiry was prompted by information recently made available to the FBI. Also, he said, some people whom he declined to name had become more cooperative.

Federal officials familiar with the case say investigators could examine whether the forgeries were instigated by U.S. citizens who advocated an invasion of Iraq or by members of the Iraqi National Congress.