Initial probe suggests that the alleged France train gunman, 25-year-old Moroccan national Ayob El Khazzani has denied any intention of waging a jihadist attack. He said he had merely stumbled upon a weapons stash and decided to use it to rob passengers.
But he was flagged as a radical Islamist by intelligence services in several countries, and French investigators are focussing on an extremist attack.
Under French law, suspects in probes related to alleged terrorism can be questioned for up to 96 hours, which means Khazzani could remain in custody until Tuesday evening.
The incident: As it happened
Armed with the weapons, the attacker exited a toilet cubicle on the high-speed train just after it crossed from Belgium into northern France.
A French passenger who happened to be there tried to disarm Khazzani -- described as "small, slim, not very strong" -- but he got away and fired at least one shot, wounding a Franco-American traveller in his 50s.
But the attack was quickly stopped when two off-duty US servicemen and their friend charged the gunman and restrained him.
"I looked back and saw a guy enter with a Kalashnikov. My friends and I got down and then I said 'Let's get him'," Alek Skarlatos, a 22-year-old member of the national guard in Oregon who recently returned from Afghanistan, told France's BFMTV.
Spencer Stone, who serves in the US Air Force, was first to the gunman, who slashed him in the neck and almost sliced off his thumb with a box-cutter.
"At that point I showed up and grabbed the gun from him and basically started beating him in the head until he fell unconscious," said Skarlatos.
His friend Anthony Sadler, a 23-year-old student at Sacramento State University, and a British business consultant, Chris Norman, then helped keep the man subdued.
Khazzani was arrested when the train with 554 passengers aboard stopped at Arras station in northern France.
French President Francois Hollande is to thank the group for their courage at the Elysee Palace on Monday.
US President Barack Obama, meanwhile, called the three Americans and congratulated them on their "extraordinary bravery".
Suspect was well-travelled
The shooting underlines the difficulties faced by intelligence services in tracking an unprecedented number of suspected Islamic extremists.
A Spanish counter-terrorism source said Khazzani had lived in Spain for seven years until 2014.
During his time in Spain, he came to the attention of authorities for making hardline speeches defending jihad, and was once detained for drug trafficking, according to the source.
Spanish intelligence services say he went to France, from where he travelled to Syria, but the suspect has reportedly denied going to the conflict-ridden country where the Islamic State group controls swathes of territory.
A source close to the French probe, meanwhile, said he "lived in Belgium, got on the train in Belgium with weapons likely acquired in Belgium. And he had identity papers issued in Spain."
French interior minister Bernard Cazeneuve said Spanish intelligence services had tipped off France over his ties to "radical Islamist movements", but it is unclear whether he lived in France at any time after leaving Spain.
German security services, meanwhile, flagged Khazzani when he boarded a flight from Berlin to Istanbul in May this year and in Belgium, Justice
Minister Koen Geens confirmed Khazzani was "known" to the country's intelligence services.
France has been on high alert since Islamist gunmen went on the rampage in January, killing 17 people in Paris, and authorities have since thwarted several other attacks.