Senator Carl Levin released a report on Thursday after a 16-month investigation by the committee's Democratic staff members accusing the Counterterrorism Evaluation Group (CEG), set up by the Pentagon after the 11 September attacks, of sidestepping the CIA and manipulating information to support the Bush administration's assumptions.
Levin's report concluded: "Intelligence relating to the Iraq-al-Qaida relationship was manipulated by high-ranking officials in the [Defence Department] to support the administration's decision to invade Iraq."
Undersecretary of Defence Douglas Feith - one of the Bush administration's strongest advocates for war against Iraq – led the now defunct CEG.
The 46-page document detailed how Feith's analysts repeatedly sought to outflank the CIA, which was much more sceptical of Iraq-al-Qaida ties.
The report found that Defence Department analysts customised presentations to different audiences - making a more aggressive case when briefing senior Pentagon and White House officials than when showing their findings to senior CIA officers.
"It was an inaccurate, exaggerated source which the CIA insisted be corrected"
Levin said distortions by analysts working for Feith fuelled many of the Bush administration's most controversial claims as it made its case for war. He noted that Vice-President Dick Cheney referred to a magazine story cataloguing many of the Feith team's controversial findings as the "best source of information" on the relationship between Iraq and al-Qaida.
"It wasn't the best source," Levin said. "It was an inaccurate, exaggerated source which the CIA insisted be corrected."
Republicans have criticised the report as being politically motivated.
"Senator Levin's report is clearly a partisan effort to influence the upcoming election rather than an attempt to correct the flaws in our intelligence community," said Senator John Cornyn of Texas, a Republican on the Senate Armed Services Committee.