How effective has the Air Marshal program been to deter terrorism on flights?

How effective has the Air Marshal program been to deter terrorism on flights?

Nico Melendez (TSA Regional Spokesperson) gives expert video advice on: Why do I have to set aside electronic equipment like my laptop and PDA?; What is the best way to travel with camera and film equipment?; What can I do if I feel a TSA employee has treated me inappropriately? and more...

It's really hard to grade how effective any element of security has been, be it explosive detection screening of bags or federal air marshals. The fact is, we haven't had a terrorist attack on the aviation transportation sector since September 11, 2001. Since September 11, 2001 the TSA has significantly increased the number of federal air marshals in our ranks. So we do see air marshalls as a very effective tool in the aviation security blanket.