"We have opened an inquiry under the anti-terrorism law ... as the suspect boarded the train in Brussels," Eric Van der Sypt, a spokesman for the prosecutor's office, told news agency AFP Saturday.
The gunman, who was armed with a Kalashnikov assault rifle, an automatic pistol, nine cartridge clips and a box-cutter, opened fire on board a Thalys high-speed train just after it crossed into France on Friday evening.
He was overpowered by a group of passengers, including two American servicemen.
French Interior Minister Bernard Cazeneuve said two people were injured on the train, which was travelling from Amsterdam and carrying 554 passengers.
The suspect, who officials say is a 26-year-old of Moroccan origin, is now being interrogated by counter-terrorist officials near Paris. A Spanish counter-terrorism source told AFP that he is understood to have lived in both Spain and France and that he travelled to war-torn Syria from France last year.
In Belgium, patrols of the high-speed Thalys rail line as well as checks and patrols at international train stations would be intensified beginning this weekend, authorities said. Baggage checks were also set to be reinforced.
Belgium's Prime Minister Charles Michel and French President Francois Hollande spoke Friday and agreed to step up cooperation on security matters between their two countries.
French investigators had turned their focus to Belgium in January after terrorist attacks on the satirical magazine Charlie Hebdo and a kosher supermarket.
Officials determined that Amedy Coulibaly, who killed four people at the supermarket in Paris, bought an automatic weapon in Belgium.
Shortly after, a raid at an apartment near the railway station in the eastern city of Verviers uncovered stockpiles of Kalashnikov assault rifles. Two suspects, who had recently travelled to Syria, were killed at the apartment in a shootout with Belgian police.
Claude Moniquet, co-director of the European Strategic and Security Centre in Brussels, says Belgium “has always been a place where it is possible to buy, legally or illegally, some weapons.”
“Belgium is one of the main points of arms trafficking in Europe because we have an important community of people coming from the Balkans, and most - 90 per cent - of the arms on the black market, especially the Kalashnikovs, come from the wars in Yugoslavia: Serbia, Bosnia, Kosovo, Macedonia and so on,” he told RFI in a telephone interview from Brussels.
“Lots of those arms come to Germany and to Belgium, and some also to eastern France. But Belgium is very special in this case. Of course you have the Antwerp port which is one of the most important ports in Europe, and which is clearly a place of trafficking, not only of arms but of drugs, diamonds, anything.”
While most arms are transported on vehicles, it would not be surprising if assault rifles travelled from Belgium to other European countries by train, as was reported by Belgian media on the weapons purchased for the January attacks in Paris.
“There is absolutely no control on the trains,” he said. “You go on the train with a bag, with a suitcase, and if you are not suspect, no one will ask you to open your luggage or your bag – no one.”
Moniquet is not convinced that security protocols will change, however.
“If you want to secure the line, you must secure each point of access. That means maybe two, three, four, 10, 15 stations, which is impossible.
“If you instead control luggage randomly, that means it will be like a plane and you must go to the station one or two hours before boarding the train, which is just unthinkable because the train must be a speedy mode of transportation. If you slow it down (passengers) will lose interest.”
Moniquet says it is likely France and Belgium will add armed police patrols to trains for additional security.
“It’s the only thing that I see that will provide security to train transport,” he said.
While no major security changes were immediately announced in other European countries, varying levels of precautions are already in place.
Long distance train passengers' bags are generally subject to a check in Spain, especially at Madrid's Atocha train station where 191 people were killed in a 2004 bomb attack.
In Italy, security checks have been increased at the main rail station since May following concerns over a heightened risk of attack. However, baggage is not subject to inspection.
On British soil only Eurostar trains, which link Britain and France, are under a security scheme that includes strict checks. However, transit police, who are sometimes armed, patrol major British rail stations.
Neither Germany nor Switzerland have heavy security measures visible to travellers on their railway systems.