Opposition Democrats said the report undercuts US President George W. Bush's justification for going to war.
It discloses for the first time an October 2005 CIA assessment that prior to the war Saddam's government "did not have a relationship, harbor, or turn a blind eye toward Zarqawi and his associates."
Bush and other administration officials have said that the presence of Zarqawi in Iraq before the war was evidence of a connection between Saddam's government and al-Qaeda.
Bush: Pursuing military effort in Iraq crucial
Zarqawi was killed by a US airstrike in June this year. The long-awaited report, said Senator Carl Levin, a member of the committee, is "a devastating indictment of the Bush-Cheney administration's unrelenting, misleading and deceptive attempts" to link Saddam to al-Qaeda.
The report, two years in the making, comes out amid a series of Bush speeches stressing that pursuing the military effort in Iraq is crucial to winning the war on terrorism, and two months before that policy will be tested in midterm elections.
The report deals with two aspects of prewar intelligence - the role of the Iraqi National Congress and its exile leader Ahmed Chalabi and a comparison of prewar intelligence assessments and postwar findings on weapons of mass destruction and Saddam's links to terrorist groups.