Follow this link to the HuffPo article about some Benjamin Franklin letters that were found by a reasearcher working in London at the time.

One of the comments about the HuffPo article pointed to the scholarly article in William and Mary Quarterly, 3d Series, Volume LXVI, Number 2, April 2009 where this find was first published.  The link leads to the 53 page article in PDF form.

Follow this link to the Newsweek article about how Ali Soufan, an FBI agent, got Abu Zubaydah to talk without torture.

From my reading of the story it seems pretty clear that the CIA takeover of the interrogation and introduction of harsh techniques cut off the flow of information.