CNN
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Hamas leader Yahya Sinwar, believed to be one of the architects of the militant group's October 7, 2023 terror attack and Israel's most wanted man, was killed in Gaza on Wednesday, according to the Israeli military.

Sinwar was one of the main targets of Israel's war against Hamas in Gaza, and Israeli officials branded him with many names, including the “face of evil” and “the butcher of Khan Younis.” Sinwar, once a very public figure, has not been seen since the October 2023 attacks and likely survived the final year of Israel's siege of Gaza by bunkering down in a vast network of underground tunnels.

In August, Sinwar became one of Hamas's most senior leaders after his predecessor Ismail Haniyeh was assassinated in the Iranian capital Tehran.

But he had long been a key player in the militant group. Sinwar joined Hamas in the late 1980s and quickly rose through its ranks. He founded Hamas's feared international intelligence agency, the Maid, and was known for using brutal force against anyone suspected of collaborating with the Israelis. Some also viewed him as a pragmatic political leader: in 2017, Hamas elected Sinwar as political chief of its main decision-making body, the Politburo, in Gaza.

Sinwar was born in 1962 in a refugee camp in Khan Younis in the southern Gaza Strip. His family was expelled from the Palestinian village of Al-Majdal – today's Israeli city of Ashkelon – during the Arab-Israeli war.

Sinwar enrolled at the Islamic University in Gaza in the early 1980s, where he studied Arabic, became involved in Palestinian nationalist student organizations, and was imprisoned for his involvement with anti-occupation activists. In 1985, before Hamas was founded, he helped organize the Maid, a network of Islamist youth that exposed Palestinian informants working with Israel. This group was later incorporated into the Hamas security apparatus of the same name.

Sinwar was sentenced to four life sentences in Israel in 1988, accused of orchestrating the murders of two Israeli soldiers and four Palestinians suspected of collaborating with Israel.

During his incarceration, Sinwar allegedly abused and manipulated fellow inmates, punished those he believed to be informants, and pressured others to go on hunger strikes.

Sinwar said he spent his years in prison studying his enemy, including learning to read and speak Hebrew at the Open University.

In 2011, he was released as part of a prisoner swap in which more than 1,000 Palestinian prisoners were exchanged for Gilad Shalit, an IDF soldier who had been held in Gaza for five years.

At the time, Sinwar called the exchange “one of the great strategic monuments in the history of our cause.” Sinwar's release was attributed to his brother being one of Shalit's kidnappers, who insisted on including Sinwar in the deal.

After his release, he returned to Gaza, where he began his rise in the militant organization and became notorious for violent treatment of suspected collaborators.

While some saw Sinwar as a militant hardliner, others saw him as a master strategist.

Fifteen years after his prison sentence, he used his knowledge of Hebrew to urge the Israeli public to support a ceasefire with Hamas in an interview with an Israeli broadcaster. “We will not recognize Israel, but we are ready to conclude a long-term ceasefire with Israel that will bring calm and prosperity to the region,” he said.

And in a rare interview with an Italian journalist in 2018, Sinwar suggested the group was ready to find a political solution, saying: “A new war is in no one's interest.”

He also alluded to the reality he and others faced in Gaza under the Israeli blockade, drawing on his own experiences in Israeli prison. “I never got out – I just changed prisons,” he said of life in Gaza.

In 2018, Hamas, under Sinwar's leadership, launched its March of Return campaign, in which Gazans protested weekly near the Israeli border, calling on Israel to lift its blockade and grant Palestinians the right to return to their ancestral villages and cities to return. The demonstrations drew international attention and support from human rights groups. At one of the protests, Sinwar applauded those who faced “the enemy that besieges us.”

As the group's political leader, Sinwar focused on the group's foreign relations and forged important relationships with regional Arab powers.

According to an investigation by the European Council on Foreign Relations (ECFR), he was responsible for restoring Hamas's ties with Egyptian leaders who were suspicious of the group's support for political Islam and for ensuring Iran continued to withdraw military funds.

Israel has publicly accused Sinwar of being the “mastermind” behind Hamas' Oct. 7 terror attack against Israel – although experts say he is likely one of several – making him a key target of its war in Gaza.

The attack was the deadliest attack in Israel's history. Hamas and other Palestinian armed groups killed more than 1,200 people, mostly civilians, and also took about 250 people hostage in the Gaza Strip.

Sinwar was considered a key decision-maker and likely the most important contact within the Gaza Strip during the intensive negotiations over the return of the hostages brought to the enclave by Hamas in the October 7 attacks. Senior figures from Israel, Hamas, the United States, Qatar and Egypt took part in the talks.

Throughout the war, Sinwar consolidated Hamas' leadership and became by far its most important figure. His influence increased after other senior Hamas officials were killed, including Mohammed al-Masri, popularly known as Mohammed Deif, the commander of the Al-Qassam Brigades, Hamas's military wing, and Deif's deputy Marwan Issa.

In 2015, Sinwar was designated a global terrorist by the US State Department and the European Union. In recent years he has been subject to sanctions by the United Kingdom and France.