You know those screenings you go through before getting on an airplane? Those screenings are conducted by the Transportation Security Administration, an agency with the U.S. Department of Homeland Security that was formed in 2001 after September 11. Their stated goal is "to prevent any terrorist or criminal activity". But, in its weekly publication of airport incidents, it has never reported an explicit attempted act of terrorism!
However, the TSA has busted hundreds of illegal aliens and drug smugglers. They have also arrested passengers for trying to bring firearms onto planes or using fraudulent travel documents. In 2008, they actually caught a U.S. Army veteran who tried to bring pipe bomb materials onto a plane, but airline officials have insisted that passengers were not in danger, because he never intended to assemble the bomb on the plane. Also, in the TSA's defense, it would be impossible to calculate the number of terrorist plots that the TSA has deterred, simply by existing.