A US passenger who tackled a gunman on a train from Amsterdam to Paris said survival instinct led him to react.
Off-duty US airman Spencer Stone said he had just woken from a deep sleep when he saw the gunman and moved to restrain him on Friday.
He overpowered the gunman with Alek Skarlatos, a member of the US National Guard, US student Anthony Sadler and Briton Chris Norman.
They are to receive the Legion d'Honneur from the French president.
Francois Hollande will present the award - France's highest honour - in recognition of their bravery, early on Monday.
A French passenger, who wants to remain anonymous, and a French-American man who was shot during the attack will receive the award at a later date.
Meanwhile, French authorities say the suspect being questioned, Moroccan national Ayoub El-Khazzani, 25, has links to radical Islamism.
He was flagged up to French authorities by their Spanish counterparts in February 2014.
He is reported to have lived in France, Spain, and Belgium and to have travelled to Syria.
Hugh Schofield, BBC News, Paris
The French are enthralled by the three Americans who acted so swiftly to stop the Thalys gunman. In their news conference Sunday afternoon at the US embassy in Paris, they came over as archetypes of American masculine virtue: handsome, strong, modest.
Deep in the French gene, there is something that responds positively to this. It is the same spirit that is so grateful - 70 years on - for the American sacrifice in the Normandy landings: a recognition of the American capacity to join moral clarity with swift, decisive action.
Recognition, but also not a little envy. The press has made much of the fact that while the three Americans - and their ally the British businessman Chris Norman - have been feted with news conferences and all the rigmarole of instant fame, the Frenchman who also played a heroic part has preferred to remain anonymous.
It is a source of national pride - and a relief - that it was not just les Anglo-Saxons who "had a go".
Mr Stone, Mr Skarlatos and Mr Sadler, have been hailed as heroes for their actions on Friday, and gave a press conference at the US embassy in Paris on Sunday.
It was the first appearance before the cameras for Mr Stone, who has been treated in hospital and was wearing a sling because of injuries to his thumb.
Asked what led him to react, he replied "survival".
"I turned around and I saw he had what looked to be an AK-47 and it looked like it was jammed or wasn't working and he was trying to charge the weapon.
"Alek just hit me on the shoulder and said 'let's go' and ran down, tackled him. We hit the ground."
Friday's incident began when a French passenger tried to enter a toilet, encountered the gunman and tried to overpower him. It is thought this passenger may have since requested anonymity.
A gun was fired and the French-American passenger was injured by the bullet. Mr Stone said he saw blood squirting out of his neck.
"I just stuck two of my fingers in the hole, found what I thought to be the artery, pushed down and the bleeding stopped," he said. "I just said 'thank God' and held that position until the paramedics got there."
Mr Skarlatos also said his initial reaction was "mostly just gut instinct", and that military training had only played a role in providing medical help and making sure there were no accomplices.
Mr Skarlatos disputed a statement the gunman made through a lawyer, that he just wanted to rob passengers because he was hungry.
He said he had a lot of ammunition and "his intentions were pretty clear", but that he appeared untrained.
"He clearly had no firearms training whatsoever."
"If he knew what he was doing, or even just got lucky... we would have all been in trouble and probably wouldn't be here today - along with a lot of other people."
Mr Stone said Friday's events had still not sunk in. "It feels very unreal. It feels like a dream."
Security aboard the high-speed Thalys service on which the incident took place is being stepped up. The trains link major cities in the Netherlands and Belgium to Paris.
Patrols and security checks will also be boosted at international train stations, and more baggage checks will be carried out.
France's security services have placed been on high alert since January when Islamist militants killed 17 people in and around Paris - including the attacks at the offices of satirical paper Charlie Hebdo and a Jewish supermarket.