We learned about an enemy who is sophisticated, patient, disciplined, and lethal. The enemy rallies broad support in the Arab and Muslim world by demanding redress of political grievances, but its hostility toward us and our values is limitless. Its purpose is to rid the world of religious and political pluralism, the plebiscite, and equal rights for women. It makes no distinction between military and civilian targets. Collateral damage is not in its lexicon. We learned that the institutions charged with protecting our borders, civil aviation, and national security did not understand how grave this threat could be, and did not adjust their policies, plans, and practices to deter or defeat it. We learned of fault lines within our government—between foreign and domestic intelligence, and between and within agencies. We learned of the pervasive problems of managing and sharing information across a large and unwieldy government that had been built in a different era to confront different dangers.

—Thomas H. Kean, government commission report,
July 22, 2004

Which event prompted the investigation that led to these findings?