PARIS — Two women were killed in southern France on Sunday by an assailant armed with a knife, in an attack that forced the evacuation of a major train station, the authorities said.

The assailant was fatally shot by a military patrol after killing the women in front of the Saint-Charles train station in the Mediterranean port city of Marseille. The attack interrupted train service and unnerved the country as it debated the government’s new security bill.

The Paris prosecutor’s office, which handles terrorism cases nationwide, said on Sunday that it had opened a terrorism investigation, but the authorities said the assailant’s motives were not entirely clear.

“This act could be of a terrorist nature, but at this hour we cannot affirm it,” said Gérard Collomb, the French Interior minister, who is in charge of domestic security.