Authorities said they had identified the corpse of Belgian national Abdelhamid Abaaoud from fingerprints in the aftermath of Wednesday's raid, in which at least two people died including a female suicide bomber after a gun battle with police.
"It was his body we discovered in the building, riddled with impacts," a statement from the Paris prosecutor said, a day after the pre-dawn raid. The prosecutor later added that it was unclear whether Abaaoud had detonated a suicide belt.
Abaaoud was accused of orchestrating last Friday's coordinated bombings and shootings in the French capital, which killed 129 people. Seven assailants died in the attack and a suspected eighth is still on the run.
Prime Minister Manuel Valls broke the news of Abaaoud's death in parliament on Thursday to applause from lawmakers who were voting to extend a state of emergency for another three months.
"We know today ... that the mastermind of the attacks - or one of them, let's remain cautious - was among those dead," Valls told reporters.
Even before last week's attacks, Morroccan-born Abaaoud, 28, was one of Islamic State's highest-profile European recruits, prominently profiled in the group's slick online English-language magazine Dabiq, where he boasted of traveling across European borders staging attacks.
The group, which controls swathes of Iraq and Syria, has attracted thousands of young Europeans, and Abaaoud was seen as a leading figure in attracting others to the movement, particularly from his home country Belgium.
He claimed to have escaped a manhunt after a raid in Belgium in 2013 in which two other militants were killed. 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.
Before the attacks, European governments thought Abaaoud was still in Syria. "This is a major failing," said Roland Jaquard at the International Observatory for Terrorism.
While quickly tracking him 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.
French officials have called for changes to the functioning of the EU's Schengen zone, which normally does not monitor the entry and exit of citizens of its 26 countries. Hundreds of thousands of Syrians have arrived in Europe as refugees in recent months, including someone who used a passport found at the scene of one of Friday's attacks.
Early on Wednesday, police swooped on the house where Abaaoud was holed up in the Paris suburb of St. Denis. Heavily armed officers stormed the building before dawn, triggering a firefight and multiple explosions.
Officials had said on Wednesday that two people were killed in the raid, including a female suicide bomber who blew herself up. Forensic scientists were trying to determine whether a third person had died. Eight people were arrested.
Two police sources and a source close to the investigation told Reuters the St. Denis cell had been planning a new attack on Paris's La Defense business district. A source close to the investigation said the female bomber who was killed might have been Abaaoud's cousin.
The victims of the deadliest attacks in France since World War Two came from 17 different countries, many of them young people out on a Friday night at bars, restaurants, a concert hall and a soccer stadium.
Islamic State says it carried out the attacks in retaliation for French air raids against its positions over the past year.
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.
The aftermath of the attacks could see common cause between Western capitals and Moscow, more than a year after the United States and European Union imposed financial sanctions on Russia over its annexation of territory from Ukraine.
Russia and the West are divided over Syria, with Moscow supporting President Bashar al-Assad and Western countries saying he must leave power to end a four-year-old civil war. Moscow launched air strikes in Syria six weeks ago and says it is targeting Islamic State, although most of its strikes have hit areas controlled by other groups opposed to Assad.
There are signs however that the recognition of a common threat since the Paris shootings and the Russian air crash could prompt more efforts to cooperate.
European Commission chief Jean-Claude Juncker wrote to President Vladimir Putin this week, suggesting closer trade ties between the 28-nation EU and a Russian-led economic bloc, linking them to progress on implementing a ceasefire in Ukraine.
In the letter, seen by Reuters, Juncker underlined the importance of good relations between the European Union and Moscow, "which to my regret have not been able to develop over the past year". He said he had asked Commission officials to study options for closer ties between the EU and the Moscow-led Eurasian Economic Union of former Soviet states.
Lithuanian Foreign Minister Linas Linkevicius said he was surprised by the letter, which he said did not reflect a common view of EU member states and made no reference to EU sanctions.
Paris and Moscow are not coordinating their air strikes in Syria, but French President Francois Hollande is due to meet Russian President Vladimir Putin in Moscow on Nov. 26 to discuss how their countries' militaries might work together.
Two days before that, Hollande will meet U.S. President Barack Obama in Washington to discuss the role of a U.S.-led coalition in any unified effort against Islamic State.
France is one of several European countries participating in the U.S.-led coalition's strikes against Islamic State targets in Iraq, and two months ago became the only European country to join strikes in Syria as well.
Obama on Thursday reiterated the U.S. position that eradicating the group was tied up with ending the civil war in Syria, which could not happen as long as Assad was in power.
"Bottom line is, I do not foresee a situation in which we can end the civil war in Syria while Assad remains in power," he told reporters in Manila on the sidelines of the annual Asia-Pacific Economic Cooperation (APEC) summit.