A decade after Sept. 11, 2001, the U.S. spends $75 billion per year on homeland security. Much of the spending has gone to aviation security. While some observers question the effectiveness of the security measures, the Department of Homeland Security notes that there is no way to determine if acts of terror would have occurred if the measures were not in place. "We know that they study our security measures, we know they're continuously looking for ways to get around them, and that's a disincentive for someone to carry out an attack," said John Cohen, DHS deputy counter-terrorism coordinator. "Another way of asking the question is: Has there been a U.S. airplane that has exploded?"