The heavily-armed alleged attacker, 25-year-old Ayoub El Khazzani, is said to be "dumbfounded" by accusations he was intending to carry out a terror attack despite being known to intelligence services in several countries for extremist links.
On Friday, he boarded a high-speed train in Brussels bound for Paris armed with a Kalashnikov assault rifle, Luger automatic pistol, ammunition and a box-cutter.
Witnesses say he opened fire, injuring a man before being wrestled to the floor by three American passengers and tied up.
"He clearly had no firearms training whatsoever. If he knew what he was doing or even got lucky and did the right thing, he would have been able to operate through all eight of the (ammunition) magazines and we probably wouldn't be here today along with a lot of other people," he said.
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The Americans revealed that they had only moved into the carriage where the attack took place because they were in search of a better Wi-Fi signal.
But he has denied any intention of waging a jihadist attack, telling investigators he had merely stumbled upon a weapons stash in park in Belgium and decided to use it to rob passengers, according to Sophie David, a lawyer assigned to his case when he was taken off the train in Arras, northern France.
"He is dumbfounded that his act is being linked to terrorism," David told BFM-TV, adding that the suspect who is believed to have lived in Belgium describes himself as homeless.
"He said he found it in the park which is just next to the Midi Station in Brussels, where he often sleeps with other homeless people.
"He says that the Kalashnikov didn't work and he was brought under control immediately without a single shot being fired," he said.
The lawyer is no longer representing Khazzani as he has been transferred to a police station near Paris for questioning.