His death, which has not yet been confirmed, would be a significant moment in Israel’s yearlong offensive against the militant group and could complicate efforts to release dozens of hostages held in Gaza.

In Gaza, no figure loomed larger in determining the war’s trajectory than the 61-year-old Hamas leader. Obsessive, disciplined and dictatorial, he was a rarely seen veteran militant who learned Hebrew over years spent in Israeli prisons and who carefully studied his enemy.

In 2008, Sinwar survived an aggressive form of brain cancer after treatment at a Tel Aviv hospital.

Prime Minister Benjamin Netanyahu released him in 2011 along with about 1,000 other prisoners in exchange for Gilad Schalit, an Israeli soldier captured by Hamas in a cross-border raid. Netanyahu was harshly criticized for releasing dozens of prisoners held for involvement in deadly attacks.

Back in Gaza, Sinwar closely coordinated between Hamas’ political leadership and its military wing, the Qassam Brigades. He also cultivated a reputation for ruthlessness. He is widely believed to be behind the unprecedented 2016 killing of another top Hamas commander, Mahmoud Ishtewi, in an internal power struggle.