The type of suicide vests used to attack a stadium and restaurants in Paris was very complex to make, indicating that the Islamic State (ISIS) is in touch with an expert bomb-maker.
Intelligence and security experts who analysed the Paris attacks last Friday, which killed at least 129 people, said the explosive vests used to blow up public places in the French capital showed sophistication and complexity in the jihadist group's terror tactics.
A former French intelligence chief, who spoke to AFP on condition of anonymity, said only a munitions expert could have helped the ISIS assemble the suicide vests.
"Suicide vests require a munitions specialist. To make a reliable and effective explosive is not something anyone can do," the former intelligence official was quoted by Newsmax as saying.
He added that only a specialist can carefully make such explosives.
"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. And it must also not blow up by accident," the ex-official added.
According to French authorities, the suicide bombs used by the terrorists who attacked Paris were made using a battery, a detonation button, shrapnel and acetone peroxyde or TATP, which is easily available at home but is very unstable to use in making explosives.
These materials indicate that the bombs were made by the expert just within France, according to the former intelligence official.
"They didn't bring these vests from Syria: the more you shake these things, the more you multiply the risks," he explained.
"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," he added.
Alain Chouet, a former director at France's DGSE external intelligence agency, agreed that the Paris bomb-maker may well be lurking around Europe.
"The explosive specialist is too precious. He never participates in attacks... So he's around, somewhere," Chouet warned.