Two people died on Sunday when the 29-year-old man grabbed the coach's steering wheel, forcing it to swerve off the A9 highway during a journey from the southern French city of Marseille to Murcia in Spain.
Two of the 41 passengers died and 30 were injured in the accident near Fitou, close to the border between the two countries.
The man, whose identity has not been made public, appeared to believe he was being taken to Spain to be killed and attempted to get the driver to stop the coach, Narbonne prosecutor David Charmatz said.
"When the driver refused, it simply confirmed his belief that he was the victim of a conspiracy," the prosecutor added.
The man is in custody undergoing further psychiatric assessment following a preliminary finding that his mental state had strongly affected, if not eliminated, his capacity for judgement, making it unlikely that he will face criminal proceedings.
Prior to boarding the coach in Marseille, the Ukrainian had attempted to enlist with the French Foreign Legion at its headquarters in Aubagne on the outskirts of the port.
"He underwent tests but the Legion didn't want him," Charmatz said.