Hamas’ announcement comes seven days after Hamas’ top political leader, Ismail Haniyeh, was killed at a residence in Tehran, Iran, in an assassination Hamas blamed on Israel—though Israel hasn’t taken credit for the move.

Sinwar, who Israeli officials have called the mastermind of Hamas’ Oct. 7 attack on Israel, grew up in southern Gaza and joined Hamas—an Islamist group founded in 1987—in the late 1980s, gaining recognition as the founder of the group’s intelligence arm, the Munazzamat al Jihad w’al-Dawa, also known as the Majd.

In 1989, Sinwar was sentenced to four consecutive life sentences for abducting and murdering two Israeli soldiers, and served in an Israeli prison—reportedly learning Hebrew there—until his 2011 release, part of a massive prisoner swap in which Israeli officials freed more than 1,000 prisoners in exchange for Israeli soldier Gilad Shalit, who had been captured by Hamas.

Since his release from prison, Sinwar has been involved in multiple rounds of fighting against Israel, including an 11-day crisis in May 2021, when Hamas and the Palestinian Islamic Jihad launched rockets at Israel and Israel conducted airstrikes in Gaza.

In 2015, the U.S. State Department designated Sinwar a global terrorist, prohibiting U.S. citizens from engaging in transactions with Sinwar—he has also been sanctioned by the U.K. and France.

Sinwar, 61, was elected Hamas’ Gaza leader in 2017 in a secret election, offering a public platform of “peaceful, popular resistance” despite a reputation as a hardliner, and was reelected to a second four-year term in 2021, months before another round of Hamas rocket attacks on Israel.

Sinwar has also gained prominence as a critic of Mahmoud Abbas, the head of the Palestinian Authority and its ruling Fatah party—a rival to Hamas that controls the West Bank but not Gaza—with Sinwar vowing in a 2022 rally to “give the chance to ignite the resistance in the West Bank.”

Sinwar vowed in that speech to come for Israel “God willing, in a roaring flood” and “with endless rockets” and “millions of our people, like the repeating tide,” according to a translation by Reuters.

While Israeli officials have cited Sinwar as the force behind Hamas’ Oct. 7 attack, Harel Chorev, a senior researcher of Middle Eastern studies at Tel Aviv University, told CNN late last year Sinwar was one of a “triumvirate” of senior Hamas officials who planned the attack on Israel.

Chorev also told CNN Sinwar is “perceived as the most senior” Hamas official, though Chorev said Sinwar is only one of several “power centers” within Hamas, which Chorev said maintains a “decentralized” organizational structure, with some senior leaders living in Qatar.

Sinwar has in the past publicly expressed caution over warring with Israel, telling Italian journalist Francesca Borri in 2018 a “new war is in no one’s interest,” adding Hamas would be open to negotiating with Israel in exchange for Israel and Egypt lifting their blockade of Gaza (that blockade has been in place since Hamas took control of Gaza in 2007).

As a result, several news outlets have reported that Israeli analysts previously believed Hamas leaders—including Sinwar—were not interested in continued fighting, an impression that ended following the Oct. 7 attack, which led to 1,200 deaths in Israel and roughly 240 hostages being taken.

IDF spokespersn Lt. Col. Richard Hecht has called Sinwar a “dead man walking” late last year, while a senior Biden Administration official said in December the White House believes Sinwar’s “days are numbered,” multiple outlets reported, adding: “He has American blood on his hands.”