The FBI's investigation began after questions were raised about a 16-word portion of President George W. Bush's January 2003 State of the Union speech when he said that Iraq was pursuing the uranium ore, part of his argument to justify the coming invasion of Iraq.
Some U.S. and foreign officials disputed the authenticity of documents that supported Bush's contention, which indicated President Saddam Hussein was seeking the uranium ore for a nuclear weapons program.
The Federal Bureau of Investigation had refused comment on the matter until an Italian news agency reported this week that FBI Director Robert Mueller sent the Italian government a letter in July with results of the bureau's two-year investigation.
The investigation "confirmed the documents to be fraudulent and concluded they were more likely part of a criminal scheme for financial gain," FBI spokesman John Miller said Friday, describing contents of the letter.
Miller did not say what led the FBI to its conclusion or identify perpetrators of the hoax.
Italian officials earlier this week identified Rocco Martino, described as a one-time informant for the Italian secret service, as the source of the forged documents, according to Italian Sen. Massimo Brutti.
Martino previously had given media interviews acknowledging his role.
Italy's spy chief, Nicolo Pollari,had any hand distributing the phony dossier, Brutti and other lawmakers who attended a private briefing said.
The session occurred after a newspaper report alleging Italy had passed the documents to Britain and the United States knowing that they were fake.
Sen. Jay Rockefeller of West Virginia, senior Democrat on the Senate intelligence Committee, said he still has unanswered questions, despite the committee's recent briefing by the FBI.
"Until I receive additional information about the thoroughness of the investigation, I cannot make a judgment on the accuracy of the conclusions," Rockefeller said.
The Niger claim also is at the center of the CIA leak investigation that led to the indictment last week of Vice President Richard Cheney's chief of staff, I. Lewis "Scooter" Libby.
Libby immediately resigned and pleaded not guilty to charges he obstructed the investigation and lied to investigators and the grand jury that has been looking into the leak of the identity of covert CIA operative Valerie Plame, the wife of Bush administration critic Joseph Wilson.
Wilson traveled to Niger in 2002 for the CIA to check out the Iraq uranium story. Plame's CIA status was exposed after Wilson accused the administration of twisting intelligence in the run-up to the war to exaggerate the Iraqi threat.