The 24-year-old IT student Sid Ahmed Ghlam's plans were exposed purely by chance after he called an ambulance saying he had been shot during an armed robbery at his Paris home, prosecutor Francois Molins told journalists.
However, police uncovered an arsenal of weapons in his car and home, and detailed plans to attack one of two churches, as well as DNA evidence linking him to the murder of a woman who was found shot dead in her car over the weekend near the capital.
Paris is still on high alert with memories fresh from a jihadist killing spree that left 17 dead in January.
Police called to the scene where Ghlam was wounded found traces of his blood in the car as well as a Kalashnikov rifle, handguns, bulletproof vests, several cellphones, a laptop and documents on "potential targets and how to carry out attacks," said Molins.
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The city prosecutor said that Ghlam had told police "far-fetched" tales of how he had received the bullet wound to his leg, such as that he had shot himself while trying to throw his weapons into the Seine river.
Analysis of his communications equipment indicated Ghlam "was in touch with another person who could be in Syria on how to carry out an attack, with the latter clearly asking him to target a church", said Molins.
Several members of his entourage and family have since been detained, some of whom sympathise with radical Islam, sources close to the investigation said.
His 25-year-old girlfriend was today taken in for questioning. The sources said she was a convert to Islam and was the only woman in her neighbourhood who wore Islamic garb.
The arrest comes some three months after Islamic extremists gunned down 12 people at the Charlie Hebdo magazine, a policewoman and four others at a Jewish supermarket in a three-day reign of terror in the French capital.