PARIS - President Francois Hollande on Monday bestowed France's highest honour on a group of Americans and a Briton who overpowered a Moroccan gunman on a crowded train, saying the whole world "admires their courage and cool composure".

Anti-terror investigators were questioning the alleged attacker, 25-year-old Ayoub El Khazzani, 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.

Witnesses said he opened fire, injuring 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 at the Elysee presidential palace, 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."

A French passenger who also tackled the gunman but chose to remain anonymous, and Eric Tanty, an off-duty train driver who happened to be on board and helped restrain Khazzani, were to be honoured too.

Mark Moogalian, a 51-year-old Franco-American professor who was shot and is now recovering in hospital, and Michel Bruet, a train conductor who warned authorities, were also to be awarded medals.

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."

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 described his son as a "good boy" who preferred "football and fishing" to politics.

"I have no idea what he was thinking and I have not spoken to him for over a year," Mohamed El Khazzani told British newspaper The Telegraph in the Spanish port city of Algeciras on Sunday.

But Sadler, 23, dismissed suggestions that Khazzani 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 Khazzani had known how to handle guns, he could have killed many people.

"He would have been able to operate through all eight of the magazines and we probably wouldn't be here today along with a lot of other people," the 22-year-old said.

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.

It is unclear where he was living after he left Spain.

His father said he went to France for a six-month work contract but was let go after just one month, and Spanish intelligence services say he went to Syria from France - a claim Khazzani denies.

French sources 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.

Khazzani boarded the Amsterdam-Paris train in Brussels, where it is believed he was living.

Stone - whose hand and eye were injured in his wrestle with the gunman and who wore a sling at Monday's ceremony - and Skarlatos are due to go to a US military base in Germany for health checks. Sadler is to fly back to the United States.