Three days after the attacks on a soccer stadium, a concert hall and numerous bars and cafes, French and Belgian security services were focused on the radical jihadist they believe was the leader of the plot, Abdelhamid Abaaoud. He is among the most prominent Islamic State fighters to have come out of Belgium.

A French official briefed on the investigation, who spoke on the condition of anonymity because he was not authorized to discuss operational details, said Mr. Abaaoud had mentioned plans to attack “a concert hall” to a French citizen who returned from Syria.

Mr. Abaaoud, this official said, had also been in contact with Ismaël Omar Mostefaï, one of the Paris attackers. Mr. Abaaoud also knew another attacker, Ibrahim Abdeslam; they were tried together in 2010 in Belgium for a minor offense.

Mr. Hollande said the attacks had been “planned in Syria, organized in Belgium, perpetrated on our soil with French complicity.”

Image Salah Abdeslam. Credit... French Police, via Agence France-Presse — Getty Images

The French authorities said Monday that they had conducted 168 raids across the country in an effort to root out possible terrorist threats. The raids extended from the Paris region to the major cities of Lille, Lyon, Marseille and Toulouse, they said. They also said they had arrested 23 people and detained 104 others under house arrest.

But a Frenchman believed to be involved in the Paris attacks, Salah Abdeslam, 26, a brother of Ibrahim Abdeslam, remained at large, eluding a series of raids conducted by the authorities in Molenbeek, the working-class Brussels neighborhood where the brothers lived.

A third brother, Mohamed, and four other men who had been detained in Belgium were released on Monday. At a news conference in Brussels, Mohamed said he did not know Salah’s whereabouts and added, “My parents are under shock and have not yet grasped what has happened.”

The man believed to be the architect of the plot, Mr. Abaaoud, who traveled to Syria last year and even persuaded his 13-year-old brother to join him there, is from the same neighborhood, Molenbeek, as the Abdeslam brothers.