For over a year, Israel’s security establishment, backed by the United States, dedicated vast resources and gathered mounds of intelligence in its hunt for Yahya Sinwar, the Hamas leader who was an architect of the Oct. 7 attacks.

But in the end, a unit of trainee squad commanders unexpectedly encountered Mr. Sinwar while on an operation in southern Gaza, according to four Israeli defense officials, who spoke on condition of anonymity because of the sensitivity of the matter.

Where was Sinwar found?

The Israeli unit was on patrol in southern Gaza on Wednesday when the Israeli soldiers came upon a small group of fighters, the officials said. The soldiers — backed by drones — engaged in a firefight, and three Palestinian militants were killed.

During the battle, Israeli fire brought down part of a building where the militants had taken cover, two officials said. As the dust cleared and they began searching the building, the Israeli soldiers noticed that one of the bodies bore a shocking resemblance to the Hamas leader, the three officials said.

It was a seemingly unlikely place to find him. Israeli and U.S. intelligence had long assessed that Mr. Sinwar — fearful for his own safety — had been hiding deep underground, surrounding himself with Israeli hostages to avoid assassination.

How did he die?

Photographs obtained by The New York Times, some of which later circulated online, show the body of a man with facial features strongly resembling Mr. Sinwar. The man’s body has several severe wounds, including to the head and leg. The photographs show that the body has several features matching those seen in archival footage of Mr. Sinwar, including distinctive moles near his eyes and crooked teeth.