French police were reportedly warned more than a year ago about the radical views of a gunman disarmed by US and British passengers on a train.
Spanish law enforcement told their French counterparts in March 2014 that Ayoub El-Khazzani had a ""relationship with radical Islam", the Spanish El Pais newspaper reported.
It also claimed the 26-year-old Moroccan, believed to have visited Syria last year, had been included on a European anti-extremism police database as far back as 2012.
Officials told The Associated Press El-Khazzani, was on the radar of authorities in France, Belgium and Spain.
He was identified by counter-terrorism investigators as the gunman armed with an AK-47 assault rifle and a handgun who opened fire on the Amsterdam-Paris train on Friday.
US Air Force serviceman Spencer Stone, aided by US National Guardsman Alek Skarlatos, Sacramento State University student Anthony Sadler and British IT expert Chris Norman risked their lives to tackle him and beat him unconscious.
Mr Stone was stabbed in the neck in the struggle but left Lille Hospital on Saturday night with his left arm bandaged and in a sling, giving a quick wave as he walked out to a black car with diplomatic licence plates.
Mr Norman, a married grandfather-of-two, said he helped the three Americans overpower the gunman who opened fire on the train because he thought he was "probably going to die anyway".
After giving evidence at Arras police station in France, he told waiting press: "My thought was 'OK I am probably going to die anyway so let's go'. I would rather die being active, trying to get him down than simply sit in the corner and be shot."
Mr Stone and Mr Skarlatos, who had returned from a deployment in Afghanistan in July, grabbed the man while Mr Sadler and Mr Norman joined them to help.
No-one had any time to think about the looming danger, according to Mr Norman who described the passengers' reactions as "very rapid reasoning".
Mr Norman, who lives in France, was facing towards the back of the train when he heard "glass breaking and then saw somebody running down the aisle to the front of the train". Then he spotted the AK-47.
He told reporters: "My first reaction was to sit down and hide. Then I heard one guy, an American say 'go get him'. Then I heard another American say 'don't you do that buddy' or something like that.
"Then I decided that perhaps this was the only chance for us to act as a team and try to take over."
France has been on edge since the attack on the Charlie Hebdo magazine and a Jewish supermarket in Paris in January, which left 17 people dead.
French authorities also told AP El-Khazzani had lived in the southern Spanish city of Algeciras, frequenting a mosque which is under surveillance there. He was transferred on Saturday morning to anti-terror police headquarters outside Paris and can be held for up to 96 hours
An official linked to Spain's anti-terrorism unit told AP the suspect lived in Spain until 2014, then moved to France, travelled to Syria, and returned to France.
Mr Norman and the two uninjured Americans were awarded a bravery medal from the local mayor. The White House claimed their actions had helped prevent "a far worse tragedy" while a No 10 spokesman said the Prime Minister praised "the extraordinary courage" of the passengers.
Among the 554 people on board was French actor Jean-Hugues Anglade, the star of Betty Blue and Nikita, who was hit by breaking glass as the alarm was sounded.