Since the end of 2009, the U.S. government has stopped more than 350 people with possible ties to terrorist groups from boarding U.S.-bound commercial flights, according to the Associated Press. Security rules were tightened after the attempted Christmas bombing of a jetliner in December 2009, when an al-Qaeda operative from Nigeria attempted to blow himself up on a flight to Detroit. Before those changes to security, hundreds of foreigners with ties to terrorist groups had been successfully flying to the U.S. every year, according to U.S. officials; only those specifically on the no-fly list were prevented from boarding planes. Others would have been questioned only once their plane had arrived in the United States, but the December 2009 attempt revealed the flaw in this plan—security officials had planned to question the bomber once he arrived in Detroit, which would have been too late.
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