French investigators Tuesday had hours left to question a Moroccan gunman who opened fire on a crowded train, only to be overpowered by a group of Americans and a Briton hailed by France's president for their "courage".

Ayoub El Khazzani, a 25-year-old who boarded the high-speed train in Brussels bound for Paris on Friday armed with a Kalashnikov assault rifle, a Luger automatic pistol, ammunition and a box-cutter, is being questioned by anti-terror investigators, though he insists he only sought to rob passengers.

Under French law, authorities can question Khazzani, who does not speak French, for four days after detaining him, a deadline that expires Tuesday evening. At that point, they are expected to either charge him or apply for an extension.

Witnesses said he opened fire, wounding a man before being wrestled to the floor and subdued by three young Americans -- off-duty servicemen Alek Skarlatos and Spencer Stone and their student friend Anthony Sadler -- and a Briton, 62-year-old business consultant Chris Norman.

Presenting them with the Legion d'Honneur -- France's highest honour -- at the Elysee presidential palace, President Francois Hollande said: "A terrorist decided to commit an attack. He had enough weapons and ammunition to carry out a real carnage, and that's what he would have done if you hadn't tackled him at a risk to your own lives."

"You have shown us that, faced with terror, we have the power to resist. You have given a message of courage, solidarity and hope."

Speaking as he left the Elysee, his medal pinned to his suit, Norman said it was "a little bit difficult to believe that it's actually happened".

"I think that one way or another, we are going to be facing this kind of problem quite a few times in the future, and I would invite you all to think about 'what would I do in that situation'.

"Act if the opportunity presents itself. Obviously you don't want to throw yourself in a situation that is completely hopeless, but act if you can."

- High alert -

France has been on high alert since extremist attacks in Paris in January left 17 people dead.

Intelligence services in Belgium, France, Germany and Spain had previously flagged Khazzani as an Islamic extremist.

But he is said to have told investigators he is "dumbfounded" by accusations he was intending to carry out a terror attack.

He said he had stumbled upon a weapons stash in a park in Belgium where he sometimes slept rough and decided to use it to rob passengers, according to Sophie David, a lawyer who was temporarily assigned to his case.

Khazzani's father has described his son as a "good boy" who preferred "football and fishing" to politics.

But Sadler, 23, dismissed suggestions that he was not trying to kill anyone.

"It doesn't take eight magazines (of bullets) to rob a train," he told reporters on Sunday.

National Guardsman Skarlatos added that if the attacker had known how to handle guns, he could have killed many people.

Stone -- who received hand and eye injuries as he wrestled with the gunman and who wore a sling at Monday's ceremony -- and Skarlatos flew to a US military base in Germany for health checks.

Stone will be nominated for the Air Force's highest model for non-combat bravery, officials said.

Sadler is to fly back to the United States.

A Spanish counter-terrorism source said Khazzani had lived in Spain for seven years until 2014. He came to the attention of Spanish authorities for making hardline speeches defending jihad and attending a radical mosque in Algeciras.

His father said he left for France for a work contract with a mobile phone operator -- a claim confirmed by the head of the firm called Lycamobile, who said he stayed two months at the start of 2014.

French intelligence sources, however, say he did not show up on their radar until May this year, when German authorities warned he had boarded a plane bound for Turkey.

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