Investigators believe the same Islamic State cell was behind the Paris attacks on November 13, 2015, which left 130 dead, and the March 22 suicide bombings on the Brussels airport and subway that killed 32 people.
Atar, a dual Belgian and Moroccan national, was already a suspect in the Brussels attacks in March.
He is a cousin of two of the attackers, the El Bakraoui brothers Khalid and Ibrahim, who took up the cause of Islamic State after years of violent crime in Brussels.
French investigators have identified an Islamic extremist who they allege coordinated the Paris and Brussels terror attacks from Syria.
Investigators, however, have yet to make such an identification, another source, speaking on condition of anonymity, told Reuters, and do not know who Bajolet was referring to. Bomb attacks at Brussels airport and on the city's metro claimed 32 lives on 22 March. He is believed to have recruited the two Iraqi men who blew themselves up outside the Stade de France during the Paris attacks, and of being the coordinator who the Brussels attackers submitted their plans to before striking.
Atar first travelled to the Iraq-Syria area in 2002 posing as a charity worker, but American security services concluded he was a fighter and he was sentenced to ten years in prison in Iraq.
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He was once the subject of a campaign by Amnesty International to free him from an Iraqi jail on health grounds.
Atar's younger brother Yassine was also arrested around the time of the Brussels attacks and their mother's home has been raided by police several times since the attacks.
On November 13, a wave of bombings and shootings killed 130 people and injured hundreds more across the French capital.
Oussama Atar's name has now been linked to both attacks in the role of co-ordinator, but not necessarily overall commander.
Salah Abdeslam, the only surviving suspect of the Paris attacks, remains in police custody.
The Algerian and Pakistani eventually reached a refugee camp at Salzburg in Austria, where the Algerian, Adel Haddadi, reportedly told authorities that Abou Ahmad acted as recruiter and organiser, providing false passports, money and a mobile phone.