It was a routine patrol for a unit of Israeli soldiers in the southern Gaza Strip. Then a firefight erupted 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 searching the building, the soldiers found a body that bore a striking resemblance to someone they had not expected to find, a man their country had been hunting for since Oct. 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 force of Israel’s military and security establishment, which had dedicated every means at its disposal to finding and killing him. Many believed he was hiding underground in Gaza and had surrounded himself with hostages taken from Israel.

In the end, the Israeli officials said, he was killed above ground on Wednesday, alongside two other militants, with no sign of hostages nearby. The Israeli authorities said they had confirmed his death on Thursday, using dental records and fingerprints. His DNA was also tested for confirmation, according to one Israeli official and the White House.

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