French police on Tuesday detained three men and two women as part of a probe into Mehdi Nemmouche, the suspect in the murder of four people at Brussels' Jewish Museum in May, a judicial source said.
Those arrested on Tuesday include people Nemmouche may have met while in detention, said a source close to the case. Further details about them were not known.
Nemmouche, 29, is a Frenchman of Algerian origin who spent more than a year fighting with Islamic extremists in Syria where he served as an Islamic State (ISIS) torturer.
He was arrested in the southern French city of Marseille shortly after an Israeli couple, a French woman and a Belgian were shot dead at the museum in central Brussels in a horrific terrorist attack.
Nemmouche at the time was on a bus from Brussels carrying a revolver and Kalashnikov rifle in his luggage.
In early December Nemmouche was remanded for another three months as the investigation continues, after he was charged with "murder in a terrorist context."
The museum shooting - the first such attack in Brussels in three decades - raised fears of a resurgence of anti-Semitic violence in Europe and of terror attacks from foreign fighters returning from Syria, as anti-Semitism skyrocketed in Europe.
According to prosecutors, a video found on a camera in Nemmouche's bag when he was arrested features a voice similar to Nemmouche's claiming responsibility for the attack.
After his arrest, reports surfaced that Nemmouche had been planning to launch a terror attack along the Champs Elysees, the Fifth Avenue of Paris, on France’s biggest national holiday.
AFP contributed to this report.