PARIS — The Paris terrorist attacks were carried out with the help of three French brothers living in Belgium, the authorities said Sunday, as they asked for the public’s help in finding one of them.
The French authorities said they were seeking Abdeslam Salah, 26, and described him as dangerous. The police warned the public: “Do not intervene on your own, under any circumstances.” Belgian officials said that his brother Ibrahim had died in the three-hour massacre and that another brother, Mohamed, was detained Saturday in the Molenbeek area of Brussels.
The carefully coordinated attacks Friday night, which killed at least 129 people and which are believed to be the work of the Islamic State, increasingly appear to have involved extensive planning by a network of men with sophisticated weapons who plotted their attack from outside the country.
The French interior minister, Bernard Cazeneuve, after meeting in Paris with his Belgian counterpart, Jan Jambon, said the attackers had “prepared abroad and had mobilized a team of participants located on Belgian territory, and who may have benefited — the investigation will tell us more — from complicity in France.”
French officials initially described eight attackers, but on Saturday night said only seven had died — six by blowing themselves up and one in a shootout with police. On Sunday, intelligence officials said they were looking for an eighth man believed to have been involved in the attacks, and hours later, the police released Salah’s name and photo.
One attacker — whose nationality is not yet known — evidently posed as a Syrian migrant. The Serbian newspaper Blic published a photograph of a passport page that identified its holder as Ahmad al-Mohammad, 25, a native of Idlib, Syria. He passed through the Greek island of Leros on Oct. 3 and the Serbian border town of Presevo on Oct. 7, officials in those countries said.
The first attacker to be identified by the authorities was Ismael Omar Mostefai, 29, a native of Courcouronnes, France, who had been living in Chartres, 60 miles southwest of Paris, and who, along with two other gunmen, killed 89 people at the Bataclan concert hall.