In hindsight, there were clues that alleged would-be "underwear bomber" Umar Farouk Abdulmutallab had become a dangerous person, and President Obama has said that it was "unacceptable" that those clues weren't spotted and acted upon.
During Abdulmutallab's transformation from the privileged son of a wealthy Nigerian banker to an al Qaeda trainee being taught to kill, he left a series of red flags. Now American and international law enforcement officials are trying to determine how Abdulmutallab was able to come so close to causing more terror in the skies.
Here is a look at the trail of clues Abdulmutallab left behind:
2005 to 2008: Abdulmutallab attends University College of London. From 2006 to 2007, the 23-year-old engineering student was president of the school's Islamic Society, when he invited controversial speakers to talk about jihad.
June 16, 2008: Abdulmutallab granted Visa to the U.S. Abdulmutallab is granted a two-year, multiple-entry visa to visit the U.S., which would expire on June 18, 2010.
August 2008 Abdulmutallab travels to the U.S. from London on an 11 day trip to Houston.
May 2009 The first alarm The U.K denies Abdulmutallab's application to renew his student visa because he applied to study "life coaching" at a non-existent college.
August 2009 U.S. Visa Gives Him Entree to Terror Training He arrives in Yemen, which admits him because he has a valid U.S. visa in his passport. He stays with an al Qaeda leader for about a month while being trained.
Nov. 11, 2009 First use of underwear bomb Somali man is arrested trying to board a commercial airliner in Mogadishu carrying a syringe and explosives in his underwear – a homemade explosive device similar to the one Abdulmutallab was carrying on Christmas Day.
Nov. 19, 2009 Clear warning Abdulmutallab's father, wealthy Nigerian banker Alhaji Umaru Mutallab, is alarmed by phone call from his son saying it would be their last contact and associates in Yemen would then destroy his phone. Mutullab feared his son was preparing for a suicide mission in Yemen and went to Nigerian officials who took the father to meet with a CIA officer at the U.S. embassy in Nigeria.
Nov. 20, 2009 Alert falls short The embassy sent a cable to Washington with the information provided by Mutallab. The CIA sent a report to the National Counterterrorism Center. Abdulmutallab's name is added to the government's terror database, but it is not added to the Do Not Fly list.
U.S. officials also did not share Abdulmutallab's name with Yemen intelligence officials.
Dec. 7, 2009 A missed opportunityAbdulmutallab left Yemen three months after his visa had expired. Yemen officials tell the Associated Press he should not have been allowed on the plane because of his visa infraction.
Dec. 16, 2009 A red flag is ignoredAbdulmutallab purchases his plane ticket at Accra Airport in Ghana with nearly $3,000 in cash. Buying expensive plane tickets in cash is one of the main alerts for possible terrorists.