French magistrates opened a formal investigation on Friday into a 24-year-old Algerian suspected of murdering a woman and planning a gun attack on churchgoers in a Paris suburb, a judicial source said.

Under French law, such a step means there are grounds for suspicion but it does not always mean a case will go to trial.

The suspect, an information technology student, was arrested last Sunday when the ambulance service raised the alarm after he called them to treat a gun wound. He appeared to have shot himself by accident.

A trail of blood led to his car and a stash of weapons and bullet-proof vests. He is suspected of killing a 32-year-old female motorist earlier the same day.

The Paris chief prosecutor said earlier this week that a search of the suspect's phones and laptops showed he was in touch with another person, possibly in Syria, who had asked him to target a church.

Authorities have said they believe one or more attacks were imminent.

France remains on high alert three months after Islamists killed 17 people in attacks on satirical weekly Charlie Hebdo, which had mocked Islam and other religions, and on a Jewish food store. Authorities say five attacks have been foiled since then.