It was a routine patrol for a unit of Israeli soldiers in the southern Gaza Strip. Then a firefight broke out and the Israelis, backed by drones, destroyed part of a building where several militants had taken cover, Israeli officials said.

When the dust cleared and they began to search the building, the soldiers found a body that bore a striking resemblance to someone they did not expect to find, a man their country had been looking for since October 7, 2023: Yahya Sinwar, the leader of Hamas.

For more than a year, as tens of thousands of Gazans were killed, Mr. Sinwar had eluded the full power of the Israeli army and security establishment, which had used all available resources to find and kill him. Many believed he was hiding underground in Gaza and had surrounded himself with hostages taken from Israel.

Ultimately, Israeli officials said, he was killed above ground on Wednesday, along with two other militants, with no sign of hostages nearby. Israeli authorities said they confirmed his death Thursday using dental records and fingerprints. His DNA was also tested for confirmation, an Israeli official and the White House said.

Mr Sinwar’s death was the biggest blow to Hamas’s leadership after more than a year of escalating violence in the Middle East, immediately plunging the war in Gaza into a new and uncertain phase. It came less than three weeks after Israeli forces killed Hezbollah leader Hassan Nasrallah in an airstrike south of Beirut, the Lebanese capital.

While some hoped that Mr. Sinwar’s death could end Israel’s invasion, Prime Minister Benjamin Netanyahu indicated that the offensive would not stop just because the engineer of last year’s deadly surprise attack on southern Israel had been killed.

“Today evil has suffered a severe blow – the mission before us is still not completed,” Netanyahu said in a statement. He said Israel remains determined to free the hostages still in Gaza, calling this an “obligation.”

He told Gazans that whoever “puts aside their weapons and returns our hostages, we will allow them to leave and live.” But he warned that anyone who harms Israeli hostages would pay with their lives.

Israeli leaders have long said they would not stop their offensive in Gaza until they crushed Hamas as a military and political force and released the hostages captured on October 7. About 1,200 people were killed in the attack and 250 were taken to Gaza. hostages. Of the 101 hostages still in Gaza, at least a third are believed to be dead.

Hamas’ attack reshaped the Middle East, triggering a devastating Israeli counterattack in Gaza, killing more than 42,000 Palestinians, turning much of the territory into rubble and leaving hundreds of thousands of Gazans facing hunger and hardship.

The October 7 attack also sparked fighting between Israel and other groups that, like Hamas, are backed by the Iranian government, and between Israel and Iran itself. These include Hezbollah in Lebanon and the Houthis in Yemen.

Hezbollah began firing rockets into Israel from Lebanon a day after the October 7 attack, leading to a year of cross-border fire. Last month, Israeli forces began heavy bombing and ground campaigns in Lebanon in an effort to stop the attacks and dismantle Hezbollah. Israel has killed many of its top commanders in addition to Mr. Nasrallah. Thousands in Lebanon have been killed in the ongoing attack, and around a million have been displaced.

In Gaza, the soldiers who unexpectedly met Mr. Sinwar on Wednesday were part of unit training to become squad commanders. After the firefight killed Mr. Sinwar and two other fighters, the Israelis found the area littered with explosives and cautiously approached the bodies. According to an Israeli official, they found money and weapons.

Photos obtained by The New York Times, some of which later circulated online, show the body of a man with facial features closely resembling those of Mr. Sinwar. The body had serious wounds, including to the head and leg.

Mr. Sinwar died after Israeli soldiers and intelligence agents tried for months to locate him and find clues but were never successful in trapping him, an Israeli military spokesman, Rear Admiral Daniel Hagari, told reporters. Mr. Sinwar’s DNA was found at some point in a tunnel a few hundred yards from where the bodies of six Israeli hostages were found six weeks ago, Admiral Hagari said. Israel collected DNA information from Mr. Sinwar during his decades-long incarceration in Israeli prisons.

News that Mr Sinwar had been killed sparked celebrations in Israel, with people gathering on rooftops and streets to cheer and wave Israeli flags, and drivers honking their horns.

For some of the hostages’ families, it was a moment of both satisfaction and fear. Many feared that their relatives would now be in greater danger.

Einav Zangauker, whose son, Matan Zangauker, was kidnapped from Kibbutz Nir Oz, made a direct plea to Mr. Netanyahu in a video on social media: “Don’t bury the hostages.”

“You have your winning image,” she said. “Make a deal now.”

Orna and Ronen Neutra, the parents of Omer Neutra, a hostage who grew up on New York’s Long Island and later joined the Israeli army, insisted that “all attention” should be focused on securing the prisoners’ release .

When news of Mr. Sinwar’s death swept through Gaza, many were stunned. At a makeshift cafe on the side of a road in Khan Younis, where Mr. Sinwar was born in 1962, people stopped to watch the news on television.

Another resident of the southern city, Rayan Raef Hamdan, 20, said she would wait for Hamas to confirm the news and hoped it was false, as were previous reports of his death.

“Since the beginning of the war, we have heard many of these rumors,” she said, adding, “We hope that God will prolong his life and save him from death.”

Other Gazans welcomed Mr. Sinwar’s death and blamed him for the hunger, unemployment and homelessness the conflict had caused.

“He humiliated us, started the war, dispersed us and made us displaced, without water, food or money,” said a 22-year-old Mohammed, who has been repeatedly displaced and asked that his surname not be used out of fear. of Hamas reprisals.

“He is the one who made Israel do this,” Mohammed said, calling Mr. Sinwar’s death “the best day of my life.”

Rezeq el-Sabti, a 44-year-old father sheltering with his family in a tent in central Gaza, said he hoped Netanyahu would now declare victory.

“Netanyahu will say to his people: ‘We killed Sinwar, who waged war against us,’” Mr. el-Sabti said, “and this gives us hope that the war will end.”

Still others said little would change with Mr Sinwar’s death. “Many preceded him,” said Rehab Ibrahim Odeh, 64. “And he is no better than those who died before.”

Iran’s state news media depicted Mr. Sinwar’s death as “martyrdom” and praised him for dying fighting Israel on the battlefield in Gaza.

U.S. officials immediately seized on Mr. Sinwar’s death to try to restart stalled negotiations aimed at reaching a ceasefire in Gaza that would free hostages there and allow more aid into the enclave.

President Biden told reporters in Berlin that he had congratulated Mr. Netanyahu and would send Secretary of State Antony J. Blinken to Israel in the next four or five days. “Now is the time to move forward,” said Biden, calling for a ceasefire in Gaza.

“It is time for this war to end and bring these hostages home,” he said.

Netanyahu’s office said in a statement that he had spoken with Mr. Biden and “agreed that there is an opportunity to reach a deal to free the hostages and that they will work together to achieve that goal.” ”

Reporting was contributed by Efrat Livni, Bilal Shbair, Aric Toler Riley Mellen, Christian Triebert, Ameera Harouda, Iyad Abuheweila, Isabel Kershner, Zolan Kanno-Youngs, Farnaz Fassihi And Abu Bakr Bashir.