Aeroplane cockpits have reinforced doors since the 9/11 attacks
By Gordon Corera
Security correspondent, BBC News
A central question for British and US authorities investigating the apparent attempt to blow up an airliner will be whether the perpetrator acted alone or as part of a wider conspiracy.
British authorities were first alerted to a possible domestic connection on Thursday night and have assigned teams to ascertain how strong that connection may be.
That will involve checking the identity of any individuals involved and seeing if they make any appearances on databases of possible suspects and extremists.
It is thought likely that the suspect had crossed the radar of UK security authorities, but not in a way that indicated he was planning any kind of attack or merited further surveillance.
In a similar way, the US is reported to have placed him on a terrorist watch-list, but not on the stricter "no-fly" list.
Nevertheless there will be questions as to how he still managed to get a valid visa to travel to the US.
Cat and mouse
There has long been an awareness that airliners are a favoured target for al-Qaeda and the nature of the device used on the plane will also be a key focus of the investigation.
Over the years, al-Qaeda has continued to seek new methods which can both evade airport security and do sufficient damage to bring down a plane - a cat and mouse game with the authorities which continues to change the way in which we fly.
After 9/11, airline cockpits were hardened with a huge expansion of checks on passengers.
In 2006 a group of young British men linked to al-Qaeda planned to use liquid explosives to bring down multiple planes but they were apprehended.
As a result, new restrictions were placed on liquids in hand luggage.
This latest attack appears to bear more similarities to the attempt by British national Richard Reid - the so-called shoe-bomber - who tried and failed to ignite his device in December 2001.
In this latest incident, the alleged bomber is believed to have had the high explosive PETN (also known as pentaerythritol) moulded to his body and then sewn into his underpants. He then tried to detonate the device using a syringe. But it failed to go off.
The nature of the device and the way it was concealed may have led to it evading current security arrangements. If that was the case then there will be an urgent need to establish how such a device can be detected.