"An individual who was inspired by the Sept. 11, 2001 attack and planes that destroyed the towers of the World Trade Center was detained by our services," Castaner said on France-2 television Thursday night.
Castaner said the person was "in the process of planning" an attack and seeking to procure weapons. A French security official said that the planning was in the early stages, but the DGSI intelligence service decided to move in and arrest the man Sept. 26 because he had started a hunt for weapons.
The man is a French resident in his 20s and had been identified by the DGSI as expressing Islamic radical views, the official said. Investigators are looking into whether he might have had accomplices. It was not known whether he had expressed allegiance to any extremist group, said the official, who was not authorized to be publicly named.
No other details were immediately made public. Castaner's surprise announcement came as the French security apparatus is under fire after a deadly attack inside Paris police headquarters. A long-time police administrator stabbed four colleagues to death on Oct. 3 in an incident prosecutors consider a possible terrorist attack. The attacker was killed by police.
He had become radicalized, and France's counterterrorism prosecutor said he had likely been in contact with members of an ultra-conservative Islamic movement. France remains on alert after a string of deadly Islamic State attacks in 2015 and 2016 that heightened the country's security posture at home and abroad.