Investigators are examining Mr Abdulmutallab's alleged extremist links
A 23-year-old Nigerian man has been charged with trying to blow up a flight from Amsterdam to Detroit.
Umar Farouk Abdulmutallab's route began in Yemen, from where he travelled to Ethiopia, Ghana and Nigeria. On 24 December, he flew from Lagos to Amsterdam, where he boarded the flight to the US.
The case triggered a worldwide aviation security review.
Was Umar Farouk Abdulmutallab known to the US authorities?
Yes. He was on a US watch-list.
His father warned US authorities in November 2009 about his son's alleged extreme views.
US intelligence agencies knew that Umar Farouk Abdulmutallab had travelled to Yemen and joined up with extremists there.
Weeks before Mr Abdulmutallab boarded the Detroit-bound jet, the US also received intelligence from Yemen that a branch of al-Qaeda was discussing an unnamed Nigerian being prepared for an attack in the United States.
How is the screening system supposed to work?
Reforms in 2004 aimed to close the gaps in intelligence-sharing among US agencies, highlighted by the 2001 attacks.
National Counterterrorism Center
operates the Tide system.
Intelligence reports from around the world are fed in and every evening selected information is downloaded into the TSDB.
The plot is said to have targeted a Northwest Airlines flight
People who are seen as a significant risk are then put on specific watch-lists, such as:
- the "no-fly" list - people not permitted to board a commercial airline for travel into or out of the US, believed to have some 4,000 names
- the "selectee" watch-list - individuals who must undergo additional security screening before being permitted to board an aircraft, thought to have about 18,000 names
- the state department's list of people who should not get a visa
So warnings about Mr Abdulmutallab were not of themselves sufficient to trigger action?
No. The single entry into Tide was not enough for his details to be added to the TSDB and therefore he was not on the no-fly or selectee lists. His name was among more than half a million.
What is Mr Abdulmutallab alleged to have carried on board? Why was it not detected?
A preliminary analysis by the FBI found that a device allegedly sewn into Mr Abdulmutallab's underwear contained the high explosive PETN - pentaerythritol. Officials say he planned to detonate it using a syringe filled with chemicals.
Security has been tightened at airports across the US
PETN was used by British "shoe bomber" Richard Reid, who is serving a prison sentence for attempting to blow up a Paris-Miami airliner in Christmas week of 2001.
Mr Abdulmutallab would have gone through security at Schiphol airport in Amsterdam, including passing through a metal detector and having his luggage screened.
Metal detectors would not pick up powder or liquid explosives concealed upon a person.
Is there a way to detect such explosives?
Airport "puffer" machines that blow air on a passenger to collect residue may detect such material.
Trained bomb-sniffing dogs or the use of a swab may also have worked.
A scan using equipment able to see through clothing would have detected something strapped to a body.
But the use of explosives moulded to the body, as is alleged in this case, have raised questions about the degree to which current airport security procedures work.
What steps have the US authorities taken in the wake of the alleged bombing attempt?
There was an immediate tightening of security announced by
the Transport Security Administration (TSA).
Technology exists to scan under clothes but there are privacy issues
- More pre-flight screening
- Passengers must remain seated during the final hour of a flight
- No access to hand luggage; possessions or blankets must not be left on laps during this hour.
The TSA subsequently announced tougher screening for travellers flying from or via 14 nations deemed a security risk. These include countries on Washington's sponsors of terrorism list - Cuba, Iran, Sudan and Syria.
Passengers from Afghanistan, Algeria, Iraq, Lebanon, Libya, Nigeria, Pakistan, Saudi Arabia, Somalia and Yemen also face extra checks, inlcuding full-body pat-down searches.
And anyone flying to the US from any other foreign country could be checked at random.
Dozens of names have been added to the watch-list and no-fly list.
What other actions has the Obama administration taken or is considering?
President Obama ordered immediate reviews. Homeland Security Secretary Janet Napolitano has been looking at aviation screening, technology and procedures.
Counterterrorism adviser John Brennan has been looking at how various watch-lists are created, how names are added and how intelligence is shared.
The initial reviews are to be completed by the end of the week.
Speaking after a meeting with his national security team on 5 January, President Obama said the intelligence community had failed to "connect the dots".
He said he wanted "specific recommendations for corrective actions to fix what went wrong" and he wanted reforms implemented immediately.
Are there likely to be further changes to airport and airline security?
The use of detection equipment at US airports is being examined.
Machines that are able to see under clothing are currently installed at 19 airports in the US and are used when passengers need added scrutiny.
The Dutch government introduced full-body scanners for passengers boarding flights to the US.
Amsterdam Schiphol Airport
has such machines, but has until now only used them on a limited basis because of issues over cost and privacy.
The British government has also announced it will start using full body scanners at UK airports.