Muhammad Deif, the shadowy leader of Hamas’s military wing in Gaza and one of the chief architects of the Oct. 7, 2023, attack on Israel that ignited a war, was confirmed by Hamas on Thursday to be dead. He was in his late 50s.

Mr. Deif, whose real name was Muhammad al-Masri, was one of the most senior Hamas leaders inside Gaza, and he was one of Israel’s most-wanted militants for decades. He survived a number of Israeli assassination attempts before the one that killed him and was lionized by some Palestinians as a symbol of Hamas’s resilience — albeit an enigmatic one.

While Israel announced in August that he had been killed, Abu Obeida, the spokesman of Hamas’s military wing, only acknowledged his death on Thursday in a recorded video statement. Both Israel and the United States considered him a terrorist with their citizens’ blood on his hands.

Mr. Deif was killed on July 13, Israel said, when its forces bombed a compound on the outskirts of Khan Younis in southern Gaza, according to the Israeli military — making him one of the most senior Hamas commanders slain inside Gaza in the war.

Mr. Obeida did not say when or where Mr. Deif was killed.

The strike that killed Mr. Deif also killed at least 90 Palestinians on the ground, the Gaza health ministry said, hitting within the Al-Mawasi area of southern Gaza, a part of a humanitarian zone designated by Israel. The health ministry does not distinguish between civilians and combatants in its death tolls.

The zone was a large expanse filled with hundreds of thousands of displaced Palestinians, many living in makeshift shelters, according to the United Nations. Israel said that Hamas was operating from the targeted compound within the zone.

Mr. Deif was “the beating heart” of Hamas’s military wing and one of its most important military strategists, said Michael Milshtein, a former Israeli intelligence officer specializing in Palestinian affairs.

There were few photographs of Mr. Deif in circulation, and he was rarely seen or heard from as he spent decades in hiding. His adopted name, “Deif,” means “guest” in Arabic, widely understood as a nod to the frequency with which he changed locations to avoid death or capture.

He led the Qassam Brigades, the military wing of Hamas, and was credited with transforming what was once a diffuse network of armed cells into a formidable fighting force with tens of thousands in its ranks.

“He’s a legendary figure in Hamas,” Ibrahim Madhoun, a Palestinian analyst close to Hamas, said before Mr. Deif’s death was confirmed by the militant group.

Born in 1965 to a poor Palestinian family, Mr. Deif grew up in the Khan Younis refugee camp in southern Gaza, not far from Yahya Sinwar, the top Hamas leader killed in October. In the 1980s, he studied at Islamic University of Gaza, focusing on the sciences, and joined Hamas around the time it was founded in the late 1980s.

He quickly rose through the ranks, developing a reputation as a master bombmaker and orchestrating a number of attacks on Israel, including a series of deadly bus bombings that killed dozens of people and derailed the peace process in the mid-1990s.

Not long after Hamas was founded, Israel imprisoned Mr. Deif for 16 months, starting in May 1989.

Mr. Deif was also held by the Palestinian Authority for a short time starting in 2000. At that time, the Palestinian Authority was a governing body with limited autonomy over parts of the West Bank and Gaza, and it was dominated by Fatah, a Palestinian faction that rivaled Hamas.

When a founder of the military wing of Hamas, Salah Shehadeh, was killed by an Israeli airstrike in 2002, Mr. Deif took the helm.

He upgraded and expanded the production and importation of rockets and helped develop Hamas’s extensive tunnel network underneath Gaza. He also commanded the so-called Shadow Brigade, which guards Israeli captives held by Hamas, and oversaw investment in new technologies like reconnaissance drones for the Qassam Brigades, according to Mr. Madhoun, the analyst.

Mr. Deif was believed to have lost an eye and suffered other serious wounds in the Israeli attempts to kill him, which often claimed the lives of others, including civilians. His ability to escape death — and oft-stated dedication to the destruction of Israel — enhanced his heroic status among some Palestinians.

Israeli and Palestinian analysts also credited Mr. Deif with transforming the Qassam Brigades into something closer to a traditional army.

Mr. Deif was one of the top names on Israel’s most-wanted list for decades, and survived more than eight attempts on his life, according to Israeli intelligence. In 2015, the U.S. State Department labeled him as a “specially designated global terrorist.”

He was the target of a 2002 missile strike by an Israeli helicopter on a busy street in Gaza City. In 2014, an Israeli airstrike killed one of his wives, an infant son and his 3-year-old daughter. During an earlier war with Hamas in Gaza in 2021, Israel’s military said it had tried to kill him several times.

That war — launched by Hamas after Israeli efforts to evict Palestinians from their homes in East Jerusalem and Israeli police raids of the Aqsa Mosque in Jerusalem’s Old City — was a turning point, Osama Hamdan, a Hamas leader based in Beirut, later told The New York Times.

Over the next few years, Hamas enhanced its military abilities.

It stockpiled about 15,000 rockets along with mortars, antitank missiles and portable air-defense systems, according to American and other Western analysts. And its leaders — Mr. Deif and Mr. Sinwar — drew up closely guarded plans for the Oct. 7, 2023, assault on Israel, as Hamas fighters trained on flying paragliders and taking hostages.

Once the Hamas-led attack on southern Israel got underway, Mr. Deif released a rare message. He declared that Hamas had decided to launch an operation so that “the enemy will understand that the time of their rampaging without accountability has ended,” speaking in a recorded address.

“Righteous fighters, this is your day to bury this criminal enemy,” he said. “Its time has finished. Kill them wherever you find them. Remove this filth from your land and your sacred places. Fight and the angels fight with you.”

The Oct. 7, 2023, attack has come to symbolize one of the greatest intelligence failures in Israel’s history and the shattering of its image as an impenetrable military power. It was the deadliest day for Jews since the Holocaust. Some 1,200 people were killed in the rampage and roughly 250 were taken hostage.

The attack ignited a devastating war that killed tens of thousands of Palestinians, including civilians. The 470-day war displaced most of the people in Gaza, led to a hunger crisis and the spread of dangerous disease, and made many Palestinians wonder if they had a future in the territory. A cease-fire came into effect on Jan. 19, but it remains unclear how long it will hold.

Israel responded by going to war against Hamas in Gaza with Mr. Deif and the other Hamas leaders in its cross hairs. In March, Israel killed his top deputy, Marwan Issa.

In November, the International Criminal Court issued a warrant for Mr. Deif’s arrest, accusing him of war crimes and crimes against humanity in connection with his role before, during and after the October 2023 attack.

The move had been seen as largely symbolic, with a slim likelihood of Mr. Deif’s ending up in custody. He had spent decades underground in Hamas’s tunnels under Gaza, according to Israeli intelligence officials, and had not been seen publicly in years.

The court also issued arrest warrants for Israel’s prime minister, Benjamin Netanyahu, and his defense minister, Yoav Gallant.

Recently, senior Israeli defense officials said the military believed that Mr. Deif had developed health problems that forced him to spend more time above ground.

That is apparently where an Israeli airstrike found him on July 13.

An Israeli unit that oversees the identification of high-value targets, staffed by operatives from military intelligence and the Shin Bet, had spent weeks observing a villa inside the southern Gaza humanitarian zone where another top Hamas lieutenant — Rafa Salameh — was believed to be staying with his family, according to the defense officials.

After learning that Mr. Deif appeared to be at the villa, the Israeli government authorized fighter jets to drop at least five precision-guided bombs on it, according to the officials.

That evening, Mr. Netanyahu said that an airstrike had targeted Mr. Deif but that there was not yet “absolute clarity” over his fate.

“His hands are steeped in the blood of many Israelis,” Mr. Netanyahu said. “At the beginning of the campaign, I laid down a rule: The Hamas murderers are dead men, from the first to the last.”