Paris Prosecutor Francois Molins said the gunman in the shooting at a high-speed train that left three passengers injured pre-meditated his attack, rebuffing his fanciful assertion of armed robbery.
Put in custody for 96 hours, the gunman, identified as Ayoub el-Khazani, appeared before a judge on Tuesday and an investigation had been opened against him for attempted murder with terrorist intent, Xinhua quoted Molins as saying.
"Investigations will aim to determine the origin of the seized weapons, the course and links of Khazani, and his sources of financing," he said.
The attack appears to have been "targeted and pre-meditated" as the 25 year-old suspect refused to catch the earlier train where seats were still available, the prosecutor said.
"He had watched YouTube files whilst already on the train in which an individual called on the faithful to fight and take up arms in the name of the Prophet," Molins said.
Khazzani injured three passengers before being overpowered by three Americans and one British.
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He was arrested when the train stopped at the Arras station, and had been under French police surveillance after Spanish security services identified him as radical Islamist.
He lived in the Paris suburb of Aubervilliers for "five to seven months" in 2014 before travelling to Austria and Germany.
He returned to Europe on June 4 from Turkey, according to Molins.