A 23-year-old Nigerian man is 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 has 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.
Weeks before he 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.
So why wasn't he prevented from flying to the US?
His name was on a US security watch-list of more than 500,000 people, known as Terrorist Identities Datamart Environment (Tide).
But there was apparently not enough information to put his name on the smaller Terrorist Screening Data Base (TSDB), which includes a no-fly list.
This meant no concerns were raised when he purchased his ticket to the US and had his passport and visa scanned.
Mr Abdulmutallab had a US visa valid until 2010. US officials said they had marked his file for a full investigation were he ever to reapply for a visa.
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.
The 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.
People who are seen as a significant risk are then put on specific watch-lists, such as:
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.
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 are the US authorities taking in the wake of the alleged bombing attempt?
There was an immediate tightening of security announced by the Transport Security Administration (TSA).
Measures include more pre-flight screening, and passengers are to remain seated during the final hour of a flight.
There will be no access to hand luggage and a ban on leaving possessions or blankets on laps during this hour.
The TSA subsequently announced tougher screening for travellers flying from or via 14 nations deemed a security risk.
The measures apply to travellers from 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.
And anyone flying to the US from any other foreign country could be checked at random.
President Barack Obama has also ordered a review of air security.
This will scrutinise how various watch-lists are created, how names are added and how intelligence is shared.
There had been criticism that the lists, set up under the previous Bush administration, were too large.
The use of detection equipment at US airports will also be examined, although this does raise questions about cost and privacy.
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. Amsterdam Schiphol Airport has such machines, but only uses them on a limited basis because of issues over cost and privacy.
Are there likely to be further changes to airport and airline security?
The Dutch government introduced full-body scanners for passengers boarding flights to the US.
Soon afterwards, the British government announced it would start using full body scanners at UK airports.
The next steps by the US administration will depend in part on the outcome of reviews ordered by the Obama administration.