Timeline of Terror: Clues in Bomber Umar Farouk Abdulmutallab's Past

Dec. 30, 2009— -- 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:

U.S. officials also did not share Abdulmutallab's name with Yemen intelligence officials.