Report of the 9/11 Commission

Permeable Borders and Immigration Controls
There were opportunities for intelligence and law enforcement to exploit al Qaeda's travel vulnerabilities. Considered collectively, the 9/11 hijackers

included known al Qaeda operatives who could have been watchlisted;
presented passports manipulated in a fraudulent manner;
presented passports with suspicious indicators of extremism;
made detectable false statements on visa applications;
made false statements to border officials to gain entry into the United States; and
violated immigration laws while in the United States.
Neither the State Department's consular officers nor the Immigration and Naturalization Service's inspectors and agents were ever considered full partners in a national counterterrorism effort. Protecting borders was not a national security issue before 9/11.

Permeable Aviation Security
Hijackers studied publicly available materials on the aviation security system and used items that had less metal content than a handgun and were most likely permissible. Though two of the hijackers were on the U.S.TIPOFF terrorist watchlist, the FAA did not use TIPOFF data. The hijackers had to beat only one layer of security-the security checkpoint process. Even though several hijackers were selected for extra screening by the CAPPS system, this led only to greater scrutiny of their checked baggage. Once on board, the hijackers were faced with aircraft personnel who were trained to be nonconfrontational in the event of a hijacking.