A "radicalised" Frenchman was allowed to board a Ryanair flight with a balaclava, knives and a gas bottle in his luggage, a French official said.
Manuel Broustail, who had been under house arrest following the Paris terror attacks, was held after the plane arrived in Fez, Morocco, where officials found the items in his luggage.
The 31-year-old had been allowed to board the flight in Nantes, western France, despite the country being on a heightened state of alert following the attacks in Paris on 13 November.
A Muslim convert, Broustail was expelled from the French military after allegedly being radicalised during a mission to Djibouti. Ahead of the Paris attacks he had organised "paramilitary style" training exercises in the city of Angers, western France, intelligence sources told French media.
Municipal authorities in the Loire-Atlantique region said that he was no longer under house arrest, so there were no grounds for preventing him boarding the flight on Sunday (6 March).
The municipality claimed that his hand luggage contained nothing out of the ordinary, and the suitcase did not trigger alarms when it passed through airport security because it did not contain explosives. It said France had flagged the man's presence to Moroccan authorities, and his arrest "was not a matter of chance".
Photos of Broustail's luggage which appeared in Moroccan media appear to show it containing at least four kitchen knives, a machete, two pocket knives, a truncheon, a balaclava and gas bottle.
In a statement, Dublin-based budget carrier Ryanair said the case was "the responsibility of Nantes airport security officials who are investigating".
Broustail had moved to Morocco with his wife last year and returned to France to sell his property, according to French investigators.