Police had been tapping the phone of Hasna Aitboulahcen as part of a drugs investigation and were able to track her down to the Saint-Denis suburb north of the French capital.
They watched the 26-year-old take Abdelhamid Abaaoud, suspected mastermind of the Nov. 13 bombings and shootings that killed 130 people, into the building where both died early on Wednesday morning.
She detonated a suicide belt during the seven-hour police assault on the building, where officials said a third unidentified person died with them. Aitboulahcen may be Abaaoud’s cousin.
Once they learned Abaaoud was in France from Moroccan officials, French police focused on Aitboulahcen, a woman with links to him whom they were already trailing.
Earlier, a police source said Abaaoud had been identified on CCTV footage recorded at a suburban metro station at the same time as the killings were in progress in central Paris.
He was seen at the Croix de Chavaux station in Montreuil, not far from where one of the cars used in the attacks was found, one of the police sources said.
In response to the attacks, police carried out raids across France for a fifth night. A bill to extend a state of emergency until February and give the police new powers goes before the upper house of the French parliament later on Friday.
So far, police have searched 793 premises, held 90 people for questioning, put 164 under house arrest and recovered 174 weapons including assault rifles and other guns, the Interior Ministry said on Friday.
Police searched a mosque in Brest in western France. Its imam, Rachid Abou Houdeyfa, who has strongly condemned the Paris attacks, achieved notoriety earlier this year after telling children they could be turned into pigs for listening to music.
Abaaoud, 28, was spotted on the CCTV tape at 10:14 p.m. (1614 ET) on Friday last week, after shootings at several cafes and suicide bombings near a packed soccer stadium, but while an attack was still under way at the Bataclan concert venue.
Abaaoud was a petty criminal who went to fight in Syria in 2013 and, until the attacks, European governments thought he was still there.
He is believed to have recruited young men to fight for Islamic State from immigrant families in his native Brussels district of Molenbeek and elsewhere in Belgium and France.
Moroccan-born Abaaoud was accused of orchestrating last Friday’s attacks in which seven assailants died. A suspected eighth person, Salah Abdeslam is still on the run.
Abaaoud was one of Islamic State’s highest-profile European recruits, appearing in its slick online English-language magazine Dabiq, where he boasted of crossing European borders to stage attacks. He claimed to have escaped a manhunt after a police raid in Belgium in 2013 in which two militants died.
Islamic State, which controls parts of Iraq and Syria, has attracted thousands of young European and Abaaoud was seen as a leading figure in luring others to join, particularly from his home country Belgium.
His own family has disowned him, accusing him of abducting his 13-year-old brother, who was later promoted on the Internet as Islamic State’s youngest foreign fighter in Syria.
Moroccan authorities also arrested his younger brother Yassine last month after he arrived in Agadir and has been held in custody since, a Moroccan security source said on Friday.
While quickly tracking Abaaoud down will be seen as a major success for French authorities, his presence in Paris will focus more attention on the difficulty European security services have in monitoring the continent’s borders.
EU interior and justice ministers in Brussels on Friday pledged solidarity with France in the wake of the attacks and agreed a series of new measures on surveillance, border checks and gun control.
“We must be implacable in our determination, we must speed up our action, otherwise Europe will lose its way,” French Interior Minister Bernard Cazeneuve said.
The 28 governments agreed to speed new legislation to share air passengers’ data, curb firearms trafficking and ensure closer checks on EU citizens crossing Europe’s external borders.
France has called for changes to the functioning of the EU’s Schengen border-free travel zone to make it tougher to travel across Europe.
Hundreds of thousands of people have reached Europe as Syrian refugees in recent months, including at least one person using a passport found at the scene of Friday’s attacks.
Belgium, stung by revelations several of the attackers were based there, has announced a 400 million euro ($430 million) security crackdown.
France has called for a global coalition to defeat the group and has launched air strikes on Raqqa, the de-facto Islamic State capital in northern Syria, since the weekend. Russia has also targeted the city in retribution for the downing of a Russian airliner last month that killed 224.