PARIS (AFP) - As fears rise over warnings of new explosives able to slip by standard airport security checks, eyes are turning to southern Yemen and one man in particular: master Al-Qaeda bombmaker Ibrahim al-Asiri.
Washington has warned that travellers flying to the United States from Europe and the Middle East are to face tighter airport checks after intelligence pointed to the new threat.
Officials have declined to say if a specific plot had been uncovered.
But experts say that if anyone could be behind the threat it's al-Asiri, a 32-year-old Saudi believed to be hiding out with Al-Qaeda in the Arabian Peninsula (AQAP) in Yemen's restive southern provinces.
Asiri, a one-time chemistry student also known as Abu Saleh, is on several most-wanted lists and has survived repeated attempts to kill him with US drones.
He specialises in building hard-to-detect non-metallic explosives, often using Pentaerythritol tetranitrate, or PETN, and chemical detonators.
He is reported to have been involved in making a bomb for the failed 2009 Christmas Day plot to blow up a US-bound airliner and an attempt to send parcel bombs containing PETN hidden in printer ink cartridges from Yemen to Chicago in 2010.
He is also believed to have designed a bomb used by his brother Abdullah, who died in a failed suicide attack on Saudi Arabia's deputy interior minister in 2009.
French criminologist Christophe Naudin, an expert on aviation security, said it was only intelligence, not traditional security checks, that prevented the Chicago-bound parcel bombs from reaching their targets.
"They went through all the checks and were only discovered because the Saudi secret services had an agent inside AQAP who was able to give them the flight number, route and even the parcel numbers," he told AFP.
"Without that human intelligence, they would have gone off." In the Christmas Day plot a Nigerian, Umar Farouk Abdulmutallab, attempted to set off plastic explosives sewn to his underwear on a flight from Amsterdam to Detroit.
He failed to detonate the explosives, was subdued by a passenger and is now serving a life sentence in the United States.
Asiri has refined his methods with every attempt and has been described by CIA director John Brennan as "a very dangerous individual, clearly somebody who has a fair amount of training and experience." "The great danger with Asiri is that he never tries the same thing twice," Naudin said.
"He learns from each of his failures and tries new things. With only 100 grams of PETN you can't necessarily destroy a plane, but it can be very dangerous." Some intelligence reports have even suggested that Asiri, working with a Syrian doctor, has been attempting to perfect surgically implanted explosive devices that would be virtually undetectable.
Experts have warned that Al-Qaeda may be seeking to carry out spectacular attacks as it faces competition for leadership of the global jihadist movement from the Islamic State (IS), the extremist group that has seized control of parts of Iraq and Syria.
Some reports have also suggested that Asiri may have switched allegiance to IS and could be preparing an attack on its behalf.