Larossi Abballa, whose brutal stabbing of a French police couple was claimed by the Islamic State group, told friends of his "thirst for blood" as long as five years ago.
Abballa was convicted in September 2013 for belonging to a jihadist cell that recruited fighters to send to Pakistan.
Previously known only for petty crime such as theft, he was arrested in 2011 on suspicion of having ties with Mohamed Niaz Abdul Rassed, an Indian national considered the instigator of the jihadist cell.
Though his phones had been tapped since February as part of a probe into jihadists heading for Syria, Paris prosecutor Francois Molins said there was no evidence he was planning Monday's attack.
Hailing from Mantes-la-Jolie some 60 kilometres (35 miles) west of Paris, Abballa -- who wore his hair long but with a short beard -- had recently launched a halal sandwich company called Docteur Food.
But police had been aware of his desire to carry out attacks in France since his arrest in May 2011 when he was around 20.
At the time, police found an appointment book at his parents' home containing a list of police stations, mosques and tourist sites in his area, seen as potential targets for attack.
"Have to start the work. I'm thirsty for blood, as Allah is my witness," he had written to a friend, investigators say.
"He seemed very motivated by the idea of waging jihad," a fellow member of the cell told investigators, describing him as "bizarre" and "mysterious".
Text messages with other cell members left little doubt about his intentions, with one source close to the investigation saying he seemed "determined to commit violent acts in France".
