The FBI has reopened an inquiry into one of the most intriguing aspects of the debate on pre-Iraq war intelligence: 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 uranium sales to Iraq. They were provided in 2002 to an Italian magazine, which gave them 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.

Those findings concerned some members of the Senate Intelligence Committee in the wake of published reports that the FBI had not interviewed a former Italian spy named Rocco Martino, who was identified as the source of the documents. The committee had requested the initial investigation.

After talking with committee members, FBI officials decided to pursue "additional work" on the case. They likely will explore the origins of the forgeries and whether the documents were created specifically to help make the case for ousting Saddam Hussein.

"This is such a high-profile issue for a lot of reasons, and we think it's important to make sure there aren't lingering questions," said an aide to Sen. John D. Rockefeller IV, D-W. Va., vice chairman of the Senate Intelligence Committee. "There's always a chance that you do a little more investigating and you uncover something you hadn't seen before or you hadn't realized."

A senior federal law enforcement official confirmed late Friday that the bureau has reopened the investigation.

Until now, the FBI's inquiry had been limited to probing whether foreign governments were involved in the forgeries, despite a broader request from Rockefeller that the FBI look into whether the forgeries reflected a "larger deception campaign aimed at manipulating public opinion and foreign policy regarding Iraq."

"I was surprised that the FBI ever closed it without coming to a conclusion as to the source," said former Sen. Bob Graham, D-Fla., who was chairman of the Senate Intelligence Committee when the Niger uranium claims first surfaced in the United States. "It looks as if it's a fairly straightforward investigation trail to who the source was. And I'm glad the FBI has resumed the hunt."

The claim that Iraq had obtained or was seeking uranium in Niger was a central part of the administration's case for war. It was mentioned in late 2002 by British Prime Minister Tony Blair and in January 2003 by Bush to illustrate the threat posed by Saddam.