The suicide vests used by Friday's 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 militants 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 attackers 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.
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"And it must also not blow up by accident."
French authorities say the vests appeared to have been made with TATP, or acetone peroxyde, 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," said the former intelligence chief.
"It's very likely he is here, in France or Europe, one or several guys who have come back from jihadist areas and who learned over there."
Three specialists contacted by AFP said it was probable the vests were made by someone outside the group that carried out the attacks.
"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 is extremely complicated.
"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.
"It's extremely worrying," said the retired intelligence chief who asked not to be named.
"Every service is on tenterhooks."
