Thanks to the courage and quick thinking of a few passengers, Umar Farouk Abdulmutallab's attempt to bring down a U.S.-bound jetliner on Christmas Day failed. So did the security system that is supposed to protect the flying public.

The 23-year-old Nigerian was allowed to board flights despite a host of warning signs that should have put security screeners on alert.

To begin with, the suspect's own father had warned the American Embassy in Lagos in October that his son had developed radical leanings. That alone should have sparked concerns that might have kept him from boarding the plane. More suspicions should have been aroused when Abdulmutallab used cash to buy a ticket to the United States, according to news reports, and boarded the flight without checking any bags.

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