The 24-year old electronics student, who is also suspected of the murder of a 32-year-old woman, was detained on Sunday after he apparently shot himself by accident and called an ambulance. He was not named.
France heightened surveillance of potential suspects and deployed troops to patrol sensitive sites after Islamist militants killed 17 people in January in attacks on the offices of the Charlie Hebdo satirical weekly and a Jewish food store.
Police found weapons and bullet-proof vests in the Algerian’s car and at his home, as well as printed material on Al-Qaeda and the Islamic State.
“Detailed documents that were found established beyond doubt that the individual was planning an imminent attack, probably on one or two churches,” said Interior Minister Bernard Cazeneuve.
He said the man had previously come to the attention of the French authorities as possibly wanting to go to Syria. Police had made checks on him in 2014 and 2015 without finding anything that would warrant further investigation.
Paris chief prosecutor Francois Molins said a search of the man’s phones and laptops “revealed that he was in touch with another person, who could be in Syria, with whom he was discussing ways to carry out an attack and who had specifically asked him to target a church.”
Le Monde newspaper said police believe that the suspect had tried on Sunday to steal the car of the woman who was later found dead and shot himself by accident during the aborted robbery. Her blood was found on his jacket, Molins said.
Molins said the Algerian had made “farfetched” comments, claiming that he had become afraid of holding so many weapons and had injured himself when trying to throw them in the river Seine, which flows through Paris.
A person close to the suspect was held for questioning on Wednesday, Molins said, without giving further details.
