Plenty of warning signs also preceded Nigerian Umar Farouq Abdulmutallab's attempt to blow up Northwest flight 253 over Detroit on Christmas Day 2009 with an underwear bomb made with PETN, a plastic explosive that was not detected by airport security.

CNN: Agencies often miss warning signs of attacks

At the Bridgeport residence where authorities say he lived most recently, agents with the FBI and local police, including members of a bomb squad, conducted a search, and investigators removed filled plastic bags.

CNN: STORY HIGHLIGHTS