Moroccan held in Paris over 2015 train attack
The source told AFP that the 36-year-old was suspected of providing "logistical help" to Ayoub El Khazzani who opened fire with an AK-47 on a Paris-bound train from Amsterdam carrying more than 500 passengers.
The attack was originally considered one of the many "lone wolf" terrorist acts to hit Europe in recent years.
Two people were wounded before the gunman was overpowered by three Americans on holiday in Europe, two of them off-duty servicemen.
Their quick action prevented what could have become a bloodbath.
The suspect, who lives in Spain, was arrested at a bus station in Paris as part of a joint French-Spanish investigation, with a statement from Madrid's interior ministry saying he was detained "as he was heading to Belgium".
"The Spanish police investigation showed this Moroccan was working with the cell which planned the attack, providing logistical support through transferring several of its members into the European Union or getting them vehicles," it said.
Spanish investigators believe he had pledged support to Islamic State's efforts to carry out attacks in EU countries.
Considered close to extremist circles, the suspect was unarmed as he was arrested by French counter-terror police, the Paris source said.
|The three Americans who prevented the attack star in a 2018
film of the incident in which they play themselves [Getty]
On August 21, 2015, Khazzani opened fire with a Kalashnikov on board the Amsterdam-Paris train following orders given by Abdelhamid Abaaoud who later went on to plan the Paris attacks in November that year in which 130 people died.
So far, French police are investigating four of Khazzani's accomplices, including Bilal Chatra who is thought to have acted as an advance scout for Khazzani in getting into Europe via the migrant trail from Syria.
Another is Redouane Sebbar, who is believed to have assisted in preparations for the shooting.
Two other men, Mohamed Bakkali and Youssef Siraj - both of whom have been charged in Belgium for helping Khazzani stage the train attack as well as organising the Paris massacre - were transferred to France in the past month.
Bakkali is seen as playing a key logistical role in the cell behind the Paris attacks and those in Brussels in March 2016, while Siraj is suspected of hosting El Khazzani at his home in the Belgian capital shortly before the Thalys shooting.
The Thalys attack was recently turned into a film by Clint Eastwood called "The 15:17 to Paris" in which the three Americans starred as themselves.
El Khazzani's lawyer has spoken out against the film, calling it a violation of her client's rights as it presents a "fictionalised" and "one-sided" view to the public as fact, according to the BBC.