The suicide vests used Friday by attackers in Paris -- a first in France -- were made by a highly-skilled professional who could still be at large in Europe, intelligence and security experts say.
All seven of the assailants who died in attacks wore identical explosive vests and did not hesitate to blow themselves up -- a worrying change of tactic for jihadists targeting France.
Unlike the attacks in London in 2005 where the bombers' explosives were stored in backpacks, Friday's assailants used the sort of suicide vests normally associated with bombings in the Middle East.
"Suicide vests require a munitions specialist. To make a reliable and effective explosive is not something anyone can do," a former French intelligence chief told AFP, speaking on condition of anonymity.
"A munitions specialist is someone who is used to handling explosives, who knows how to make them, to arrange them in a way that the belt or vest is not so unwieldy that the person can't move," he added.
"And it must also not blow up by accident."
But a key question about Friday is why the three attackers outside the Stade de France blew themselves up in near-deserted areas, killing only one person.
"It makes no sense," said a police source, speaking on condition of anonymity.
"If you wanted to cause carnage, you act at the moment when spectators are entering or leaving the stadium."
Flowers are placed on the doors of the Comptoir Voltaire cafe at the site of an attack on November 15, 2015 in Paris, after a series of gun attacks occurred across the city.
Only an hour earlier, a bombing could have caused dozens of fatalities and triggered a deadly stampede.
The most likely explanation, said the ex-intelligence chief, is that the attacks were timed to coincide precisely with those in central Paris.
"They were maybe not too smart and even though they weren't in the right position, they blew themselves up at the agreed time," he said.
'Not cannon fodder'
French authorities say the vests appeared to have been made with TATP, or acetone peroxide, that is easy for amateurs to make at home but is highly unstable.
The vests also included a battery, a detonation button and shrapnel to maximise injuries.
"They didn't bring these vests from Syria: the more you shake these things, the more you multiply the risks," he said.
A security guard stands at the headquarters of the General Directorate for External Security (DGSE), France's external intelligence agency, in Paris on June 4, 2015.
"It's very likely he is here, in France or Europe, one guy, or several guys who have come back from jihadist areas and who learned over there."
Three specialists contacted by AFP agreed he was probably not among the attackers.
"The explosive specialist is too precious. He never participates in attacks," said Alain Chouet, a former director at France's DGSE external intelligence agency. "So he's around, somewhere."
"The bomb-maker is not cannon fodder," added Pierre Martinet, another former DGSE official who now works in corporate security.
"He's there to make more suicide vests and allow other guys to carry out actions."
Making a vest cannot be simply learned over the Internet.
"It can't be done in a couple of days," said the former intelligence chief. "It takes weeks of training, and you have to work under the watch of a 'master'. It's meticulous work."
On the eve of the UN global climate conference in the northern suburbs of Paris later this month, followed by New Year's celebrations and next year's Euro 2016 football championships, concerns are high about another strike.
"It's extremely worrying," said the retired intelligence chief who asked not to be named.
"Every service is on tenterhooks."