But leaked excerpts from the report due for release on Wednesday indicate the first official acknowledgement from the US military that there was no "direct operational link" with al-Qaeda, according to the McClatchy Newspapers group.
The original reason for going to war - that Saddam had weapons of mass destruction that posed an imminent threat - has also lost credibility, with no such weapons found five years after the invasion.
Besides the hundreds of thousands of documents, the study's researchers also used thousands of hours of interrogations of former senior officials in Saddam's government now in US custody.
The study does indicate that Saddam Hussein did much to support "terrorism" in the Middle East and used it "as a routine tool of state power".
His government, the study says, had a programme for the "development, construction, certification and training for car bombs and suicide vests in 1999 and 2000".
But "the predominant targets of Iraqi state terror operations were Iraqi citizens, both inside and outside of Iraq" who were seen as Saddam's enemies, the report says.