Monograph on Information Sharing prepared for the 9/11 Commission is released:

http://www.fas.org/irp/eprint/wall.pdf

Legal Barriers to Information Sharing:
The Erection of a Wall Between Intelligence and Law Enforcement Investigations
Commission on Terrorist Attacks Upon the United States
Staff Monograph
Barbara A. Grewe
Senior Counsel for Special Projects
August 20, 2004

And if you want to skip to the end....

It is clear, therefore, that the information sharing failures in the summer of2001 were not the result of legal barriers but of the 'failure of individuals to understand that the barriers did not apply to the facts at hand. Simply put, there was no legal reason why the information could not have been shared.