Five suspects were interrogated on Tuesday for their possible connection to a failed bomb discovered in an apartment building in a posh neighborhood in the French capital Paris.
Judicial sources said the explosive device included two gas canisters inside the building in the affluent 16th district of western Paris and two outside, some of them doused with petrol and wired to connect to a mobile phone.
Interior Minister Gerard Collomb said one of those arrested was on an intelligence services list of “radicalized” persons.
Asked to explain how someone under surveillance could carry out an attempted attack undetected, he said radicals usually have "friends, networks that can carry out the act," people who don't show outward signs of radicalization "but are ready to help."
He said the incident shows that the threat against France remains "extremely big."
"Blowing up a building in a chic neighborhood of Paris — is this not a sign that no one is safe? This doesn't happen just in suburbs in working class neighborhoods," he said.
“We are still in a state of war,” Collomb, speaking after a Sunday attack in which a knifeman killed two women in Marseille, told France Inter radio.
It was unclear why the device was planted at the location where it was found as there was no obvious target living there, the judicial sources said.
More than 230 people have been killed in France in attacks by extremist militants over the past three years. The ISIS terrorist group, whose bases in Syria and Iraq are being bombed by French war planes, has urged followers to attack France.
Most of those killed died in attacks by extremist gunmen and suicide bombers in Paris in 2015 and when a man drove a large truck into crowds in the Riviera resort of Nice in 2016.
Since then, there has been a string of attacks perpetrated by lone assailants, often targeting police or soldiers.
“The threat is changing form,” said Collomb.
A counter-terrorism investigation is also under way after the attack on Sunday, where a knifeman slit the throat of one of his victims and killed her cousin before being shot dead by soldiers at a train station in the southern port city.
A separate inquiry has been opened to establish why the man, who had been briefly detained by police the day before in the city of Lyon had been released.
France declared a state of emergency in late 2015 after the Paris attack by gunmen and suicide bombers, giving police special search and arrest powers to combat would-be terrorists.
New legislation was due to be put to a vote in parliament on Tuesday to make many of those emergency measures permanent.
Collomb stressed the importance of the counter-terrorism law as critics say it tramples on individual liberties and puts France in a permanent state of emergency.