PARIS — The Paris terrorist attacks were carried out with the help of three French brothers living in Belgium, the authorities said yesterday (Nov 15), 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 his brother Ibrahim had died in the three-hour massacre and 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, Mr Bernard Cazeneuve, after meeting in Paris with his Belgian counterpart, Mr Jan Jambon, said the attackers had “prepared abroad and had mobilised 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. Yesterday, 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.

The Salah brothers lived in Molenbeek, an impoverished section of Brussels that is mostly populated by immigrants from the Arab world and that has been linked to violence. “We don’t have control of the situation in Molenbeek at present,” Mr Jambon told VRT, a television channel.

Amedy Coulibaly, who was involved in the deadly assault on a Jewish supermarket in January, is believed to have bought weapons in Molenbeek, as is Mehdi Nemmouche, a Frenchman who targeted the Jewish Museum of Belgium in 2014, killing four people. Ayoub El Khazzani, a Moroccan who was thwarted in his attempt to attack passengers on a high-speed train travelling to Paris from Amsterdam, is also thought to have lived there for a while.

“I notice that each time there is a link with Molenbeek,” Prime Minister Charles Michel of Belgium said yesterday. “This is a gigantic problem”.

Crucial, if sparse, details about other attackers came into view yesterday.

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. It was not clear whether the passport was authentic; the civil war that has sent millions of Syrians fleeing and fuelled the rise of the Islamic State has also created a large black market for forged Syrian passports.

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.

Mostefai was the middle of five children born to an Algerian father and a Portuguese mother, and he once worked at a bakery, according to a former neighbour at the housing development just outside Chartres where the family used to live.

“It was a normal family, just like everybody else,” said the neighbour, who spoke on the condition of anonymity. “He played with my children. He never spoke about religion. He was normal. He had a joie de vivre. He laughed a lot”.

For reasons that are unclear, Mostefai changed. “It was in 2010, that’s when he started to become radicalised,” the neighbour said. “We don’t understand what happened”.

As the authorities continued to examine Mostefai’s motivations and background, other clues emerged from official accounts in France and Belgium.

Two vehicles used in the attacks had been rented in Belgium early last week, the federal prosecutor for Brussels announced yesterday. One of them, a grey Volkswagen Polo, was abandoned near the Bataclan after being used by the three terrorists who died there.

The other, a black Seat Leon, was found early yesterday in the eastern Paris suburb of Montreuil. Three Kalashnikov rifles were found inside it. The vehicle may have been used as a getaway car for the shooters at restaurants in central Paris.

The Belgian authorities announced that they had detained seven men. Abdeslam Salah and two other men passed through a roadside check in Cambrai, France, at 9.10am. Saturday, while on the A2 highway heading to Belgium. They made their way to Molenbeek, where the authorities seized the car Saturday afternoon and arrested Salah’s companions and, separately, his brother; it is not clear how Salah got away.

The revelations that at least four men involved in the plot — the Salah brothers, and Mostefai — were French citizens were likely to exacerbate long-standing fears in France about the place of Muslim immigrants and converts in French society, 10 months after a smaller set of deadly attacks on the satirical newspaper Charlie Hebdo, on a kosher grocery and against a police officer.

Mostefai was one of three hostage-takers at the Bataclan, and was identified based on a print from his severed finger.

Mostefai grew up around Chartres, where he lived until 2012. According to the Paris prosecutor, Franois Molins, he was arrested in connection with a series of low-level crimes from 2004 to 2010 and had been under surveillance since 2010, having been flagged in a French security services database as someone who had fallen under the influence of extremist Islamist beliefs. NEW YORK TIMES