Do air marshals really help?

Three days after the attempted Christmas bombing of Northwest Flight 253, President Obama announced that federal air marshals would ride shotgun on more flights to and from the U.S. Armed, highly trained and unobtrusive, thousands of marshals are currently flying U.S. skies. But, as I note in the history of air marshals I wrote for this week's mag, whether they can prevent airborne attacks is debatable.