Five people remained in custody Tuesday after being detained by French authorities over an apparent failed bombing attempt in an apartment building in a chic Paris neighborhood.

Paris police said a neighbor alerted authorities to suspicious activity early Saturday in the building in the 16th arrondissement, or district. A judicial official said an explosive device was found and deactivated and that counterterrorism prosecutors have opened an investigation. He was not authorized to be publicly named discussing an ongoing case.

One of the suspects was under government surveillance for radicalism, Interior Minister Gerard Collomb said Tuesday.

Asked to explain how someone under surveillance could carry out an attempted attack undetected, Collomb 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.

The failed bombing came the same weekend as a deadly stabbing in Marseille by a man who used multiple aliases. His motives are unclear, though it was claimed by the Islamic State group.

Collomb also stressed the importance of a new counterterrorism law expected to win parliamentary approval Tuesday. Critics say it tramples on individual liberties and puts France in a permanent state of emergency.

“We are still in a state of war, even if the Islamic State group has seen military defeats,” he said.

(AP)