Anyone with an interest in the gritty details of the planning behind the 9/11 attacks should read Edward Jay Epstein's summary of Spanish Judge Baltazar Garzon's investigation into the connection between the hijackers and a Spanish al Qaeda cell.
In an interview, Mr. Garzon explained to me through an interpreter that the support of the Spanish cell began in the early days of the plot and continued up until the attack. He described evidence that ranged from video tapes that Spanish police had confiscated from the home of one of the Spanish conspirators, which methodically surveyed the twin towers of the World Trade Center from five different angles in the late 1990s, to a phone call intercepted by Spanish intelligence in August 2001 (at a time when the hijackers were buying tickets on the planes they planned to commandeer), in which an operative in London informed Yarkas that associates in "classes" had now "entered the aviation field," and were beheading "the bird." After drawing a diagram for me on a blackboard of how the Spanish cell connected to Atta's and Binalshibh's recruiters in Germany, he said it was "supporting the operation at every level."
As with most of what the 9/11 Commission's work, their investigation into the planning behind the hijackings is turning out to be woefully inadequate. We may never know the whole picture, but it's naive to accept on faith that the 19 hijackers were acting in isolation from America's other enemies.