The alleged terror incident aboard a passenger flight from Amsterdam to Detroit has raised questions as to how a Nigerian man carried explosives through stringent security measures.
Umar Farouk Abdulmutallab has been charged with attempting to destroy a passenger plane after he detonated a device on board a jet on Friday.
Authorities in the United States are investigating whether Abdulmutallab had any connections with terrorist organizations or was acting alone.
With Dutch officials scrutinizing security procedures at Amsterdam's Schiphol airport that allowed the 23-year-old man to smuggle the explosives on the aircraft -- here are some of the key questions still hanging over the incident.
Where did the explosives come from?
The man charged with igniting the device claims he obtained the explosives in Yemen, along with instruction on how to use it. He carried these onto a flight from Lagos, Nigeria, to Amsterdam, Netherlands where he transferred to Northwest Flight 253 to Detroit.
Was he on any security watch lists?
U.S. authorities say they were contacted by the man's father ahead of the attack, but whether he was placed on any so-called No-Fly list maintained by the United States is unclear. Dutch officials say the passenger's name appeared on a manifest supplied by the airline, which was passed to U.S. authorities, who cleared the flight to depart.
Were the proper security checks performed?
Yes, say Dutch authorities, who say he passed through normal procedures and that "security was well-performed." He is known to have passed through a metal detector and his luggage was X-rayed. Extra attention is normally applied to passengers arriving from Nigeria because of concerns over fraud and smuggling. Nevertheless airports around the world have stepped up security procedures in the wake of the incident, increasing pat-downs and secondary searches.
So how did the explosives get through?
Dutch authorities are at pains to point out that if the passenger had powders concealed upon his person or secreted bottles of liquid somewhere else, they certainly would not have been picked up by the metal detector.
Is there any way of detecting these kind of explosives?
A preliminary FBI analysis indicates the device contained PETN, also known as Pentaerythritol Tetranitrate, a highly explosive chemical. Experts say this would have been picked up using a swab commonly used in secondary screening. A body scan, particularly the new 3D imaging scanners being trailed at some airports, would also have spotted something strapped to his body, even in the crotch area.
Why were there no U.S. air marshal security officers on the flight?
U.S. Homeland Security Secretary Janet Napolitano told CNN federal air marshals are not posted on all flights. She said this was not due to budgetary constraints, but down to standard procedures, which see marshals posted randomly on certain routes.