Security was stepped up on major European rail services as fears of "blowback" attacks by jihadis returning from Syria escalated after a gunman known to the intelligence agencies opened fire on a high-speed train bound for Paris.

The lone gunman is thought to be a 26-year-old Moroccan, known to Belgian, French and Spanish authorities, who travelled to fight in Syria last year, prompting speculation the attack was linked to Isis (Islamic State).

French authorities were questioning the suspect at their counter-terrorism headquarters outside Paris as details continued to emerge of how the heavily armed gunman was overpowered by passengers on the train in France.

A Briton and two US servicemen, one of whom suffered knife wounds in the struggle to subdue the Kalashnikov-carrying attacker, were among those who wrestled him to the ground after he had boarded in Brussels.

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Within hours of the thwarted attack, security measures on the high-speed Thalys trains between France, Belgium, the Netherlands and Germany were upgraded with increased patrols and baggage checks at stations.

French media named the gunman as Ayoub el-Khazani and said he was known to intelligence services as a radical Islamist who travelled to Syria last year. France's Interior Minister, Bernard Cazeneuve, said that if the details the suspect had given proved correct, then he was a suspected Islamist militant.

Belgium's prime minister's office later confirmed that the gunman was known to Belgian authorities and they had been informed by Spain that he was a security risk. It also emerged the north African suspect may have lived in France before travelling to Syria and returning to France.

Before his attempted attack he travelled to Belgium's busiest station - the terminus of the Eurostar rail link to Britain, Brussels' Gare du Midi - to board the train.

Security experts have repeatedly warned of "blowback" - the term used to describe the rebound of violence from one area to another - and say the terrorist threat posed by European jihadis who have fought in Syria or Iraq returning home is significant. A spokesman for Europol, the EU's police agency, said Europeans returning from Iraq and Syria presented the "most serious terrorist threat Europe has faced since 09/11".

The agency's latest assessment indicates as many as 5000 Europeans have joined the fighting in Syria, posing a potentially significant risk to their homelands.

This includes around 600 Britons, 1200 French nationals and at least 350 Belgians.

Until confirmation of the gunman's identity, the only case of Isis "blowback" in Europe has been Mehdi Nemmouche, a 29-year-old French national of Algerian origin who has been charged with attacking the Brussels Jewish Museum in May 2014.

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Nemmouche had spent more than a year in Syria, and police found links to Isis. Investigators in France will be seeking to establish if the train attacker had also joined the extremist group in Syria and Iraq.

Cazeneuve said that Spanish authorities had initially alerted French intelligence services in February 2014 to Khazani because of his connections to the radical Islamist movement.

In response, French authorities placed him on the country's state security "S-list" which identifies suspects noted for the "links with terrorism", although not all are placed under surveillance.

Counter-terrorism expert Raffaello Pantucci said there was a high probability that the attacker had forged links to Isis, and Claude Moniquet, the chief executive of the European Strategic Intelligence and Security Centre, agreed that the attacker was "very probably linked to Islamic State".

Others pointed out the similarities with the recent attack in the Tunisian beach resort of Sousse in which a gunman, also armed with an AK-47, killed 38 people.

French newspaper La Voix du Nord claimed that the suspect may have had connections to a group involved in a suspected Islamist shooting in Belgium in January.

On January 8, 24 hours after the attacks on Charlie Hebdo in Paris, Belgian police killed two suspected Islamic extremists and arrested a third, who were allegedly on the point of carrying out an attack.

How the train suspect was subdued

1 Man seen with AK-47

A Frenchman who has not been identified was the first person to discover the attacker in the train restroom - with a Kalashnikov slung over his shoulder, according to French Interior Minister Bernard Cazeneuve. He "courageously tried to tackle him". AFP says a Franco-American traveller in his 50s then clashed with the man and was shot and wounded. Three US passengers heard a gunshot and breaking glass and watched a train employee race past them with the gunman in pursuit, "cocking his gun".

2 Gunman tackled in carriage

Spencer Stone, a US Air Force member, was the first to reach the gunman as the Kalashnikov jammed. "I saw a guy entering the train with an AK [Kalashnikov rifle] and a handgun and I just looked over at Spencer and said, 'Let's go, go,'" said Stone's friend, National Guardsman Alek Skarlatos. "Spencer got to the guy first."

3 Tourist wrestles him to floor and is slashed by box-cutter

The gunman shouted: "Give me back my rifle". In the struggle Stone was slashed in the neck and hand with a box-cutter. The gunman also had a Luger automatic pistol and nine cartridge clips. Stone helped staunch the flow of blood from the other injured man before he became too weak himself.

4 Three men rush into the fray

Skarlatos, Sacramento State University student and friend, Anthony Sadler, and British business consultant Chris Norman helped subdue the gunman. They held him down, beat him up until he passed out and then tied him up by hands and feet. Skarlatos was right behind Stone as he charged the attacker, grabbing the man's assault rifle and butting him on the head with its muzzle. Skarlatos took a handgun from the man and threw it away.

5 Passengers keep guard over tied-up gunman

Norman said: "My thought was, okay, I'm probably going to die anyway, so let's go. I'd rather die being active, trying to get him down, rather than simply sit in the corner and be shot. Once you start moving, you're not afraid any more." Skarlatos picked up the AK-47 and checked the other carriages in case there was another gunman. "I noticed when I removed the round in the chamber of the AK that the primer had been struck, which means he pulled the trigger on the AK," he said. "The primer was just faulty, so the gun did not go off, luckily, and he did not know how to fix it, which was also very lucky. When I cleared the handgun I also noticed that there was no magazine in it - so he had either dropped it accidentally or did not load it properly. He was only able to get what appeared to be one shot off with the handgun."

6 Gunman is bundled on to platform and arrested

A French actor broke the alarm on the train, which was met by dozens of police officers when it stopped at Arras. Stone left hospital in Lille yesterday, his left arm bandaged and in a sling. The wounded French citizen was shot in the torso and is in a serious but stable condition.

Into the fray

Spencer Stone:

The Airman 1st Class, 23, joined the military after school. He had always dreamed of becoming a soldier, said his mother. Stone is based in the Azores but was on holiday.

Alek Skarlatos: The National Guard reservist ended a stint in Afghanistan in June and was in Europe for a month with friends. Skarlatos spent his teens in Roseburg, Oregon, after moving there from California with his father. He was a keen lacrosse player at school and joined the National Guard three years ago.

Anthony Sadler: The Sacramento State University student is 23. "He leaves here a young man on an excursion and he comes back France's national hero. I'm still wrapping my head around that," his father, Anthony Sadler snr, said.

Chris Norman: The Uganda-born Briton, 62, grew up in the south of France and has spent four years living in England, where he has family. He is married with two children, one of whom is in the French Navy, and two grandchildren. He has lived in France for the past two decades, based in Nimes. He helps entrepreneurs from Africa to find financing in Europe.

- Daily Mail, AFP, PA, Telegraph Group Ltd, Observer