French investigators think they know who masterminded the deadliest terrorist assault in peacetime France: Abdelhamid Abaaoud, a 20-something Belgian who joined the ranks of the Islamic State group a few years ago.

The son of a Moroccan shopkeeper, familiar to the intelligence community for his high-profile presence on social media, Abaaoud is being studied by investigators as the man who orchestrated the seven attacks in the Paris area Friday that left at least 129 people dead and hundreds more wounded, the prosecutor's office said. Abaaoud, who Le Monde newspaper says is 28, is also linked by French officials to a failed assault on a Paris-bound high-speed train in August and a plot to attack a church in the city in April.

Belgian security officials began tracking him in March 2014 after he appeared in a video behind the wheel of a pickup truck dragging mutilated bodies to a mass grave in Syria. About six months later, photos surfaced on the Internet indicating he had lured his 13-year-old brother Younes to the war zone.