PARIS (Reuters) - A suspected Islamist gunman who opened fire aboard a high-speed train travelling through northern Europe but was overpowered by three U.S. citizens before he killed anyone goes on trial on Monday in the French capital. Moroccan national Ayoub el Khazzani was heavily armed when he launched his attack moments after the train crossed into France from Belgium on Aug. 21, 2015. Khazzani is accused of attempted murder with intent to commit terrorism and association with terrorist networks, among other charges. He faces life in jail if convicted. A French citizen was the first to tackle Khazzani after a gunshot rung out in a carriage before the three Americans, two of whom were in the military at the time, rushed the attacker and disarmed him. A judicial source said Khazzani had confessed to planning the attack to investigators but had said he only intended to attack U.S. soldiers on leave, and not civilians. He told investigators he decided against the attack at the last second but that it was too late to avoid the confrontation with passengers, the source added. France was at the time still recovering from the trauma of attacks in Paris seven months earlier against the satirical magazine Charlie Hebdo and a kosher supermarket. Three months later, a squad of jihadists would strike again, killing 130 people in coordinated attacks on the capital.