Belgian police carried out raids in Belguim and took two people in for questioning on Sunday morning in concert with the investigation into 29-year-old Mehdi Nemmouche, who was arrested Friday under suspicion of carrying out a recent attack in a Jewish Museum in Belgium killing four, including an Israeli couple from Tel Aviv.
"An intervention was conducted Sunday morning in the Kortrijk region and two people are being questioned by the police," said federal prosecutor Frederic Van Leeuw. "The people are not under arrest. We want to see if they are involved or not," he said.
Police in the southeastern French city of Marseille arrested Nemmouche on Friday after he arrived on a bus from Amsterdam, Paris Prosecutor Francois Molins told reporters. The suspect had an automatic weapon like that used in the Brussels attack, and ballistics analyses were under way to determine if it is the same weapon, Molins said.
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The suspect's weapon was wrapped up in a white sheet scrawled with the name of the Islamic State of Iraq and the Levant, an extremist group fighting in Syria, Molins said. He said the suspect had spent about a year in Syria, though it was unclear if he had taken any direct role as a fighter in the county's civil war.
Fears have been mounting in European countries that the hundreds of European radicals who are joining the fight in Syria against President Bashar Assad could stage attacks when they get home.
"The new elements in this investigation (those taken in for questioning)d raw attention once more to the problem of the 'returnees' -in other words the people going to Syria to participate in combat and return afterward to our country," said Van Leeuw. "All European countries are confronted at this moment with this problem."
Nemmouche, who has so far remained silent under police interrogation, has reportedly been a guest of French prisons on five different occasions for crimes including robbery.
"During his last stay in jail he was noticed for extremist (Islamic) proselytism," Molins said. "On Dec. 31, 2012, three weeks after he was freed, he travelled to Syria."
"He spent over a year in Syria, where he seems to have joined the ranks of combatant groups, jihadist terrorist groups."
Beginning Friday at mid-day, Nemmouche's custody could last 96 hours, that is to say until Tuesday or 144 hours until Thursday, if the investigators had to invoke an imminent terrorist threat.
The suspect was also reportedly arrested in possession of a Go-Pro camera which he reportedly tried to use in the attack by attaching it to his shoulder. Van Leeuw said that the camera had malfunctioned however and didn't record any of the attack.
Van Leeuw added however, that a video found after his arrest shows his weapons and clothes, and includes his voice claiming responsibility for the attack.
The Brussels killings, which came on the eve of European parliament elections in which far right parties had a strong showing, led Belgian officials to boost their anti-terror measures, and raised fears of rising anti-Semitism.
The organized attack was filmed by security cameras showing the perpetrator calmly entering the premises of a Jewish museum and firing on civilians on the lobby.
Jewish leaders and communities throughout the world were quick to condemn the attack as having anti-Semitic motivations while French and Belgian protestors held rallies in solidarity with the victims.
Meanwhile French President Francois Hollande promised Sunday to "fight" homegrown radicals who come home from Syria with violent plans.
Hundreds of people have left from France alone to fight in Syria's 3-year-old civil war with Islamic extremists. The French government recently introduced new measures to try to stop disaffected youth from leaving in the first place, and better track those who go to Syria and come back.
Hollande said those efforts would be "amplified" in the coming months, without elaborating.
"The whole government is mobilized to follow the jihadists, and prevent them from being able to cause harm" especially when they come home to France or elsewhere in Europe, Hollande said on an official visit to Normandy.
The AP contributed to this report.