Hamas leader Yahya Sinwar killed by IDF, Israel says

Hamas leader Yahya Sinwar was killed by Israeli forces in the Gaza Strip on Thursday, Israel Foreign Minister Israel Katz said.

He has been credited as the mastermind behind the Oct. 7, 2023, attack on Israel that led to the deaths of 1,200 people, the worst terrorist attack in Israel's history.

"The master murderer Yahya Sinwar, who is responsible for the massacre and atrocities of October 7, was killed today by IDF soldiers," Katz said in a statement. "This is a great military and moral achievement for Israel and a victory for the entire free world against the evil axis of extreme Islam led by Iran."

Israeli authorities said they had been pursuing Sinwar for a year and that he had been hiding "behind the civilian population of Gaza, both above and below ground in Hamas tunnels in the Gaza Strip." The Israel Defense Forces and Israel Security Agency said their operations in recent weeks in southern Gaza restricted his movement and "led to his elimination."

“Sinwar died while beaten, persecuted and on the run -- he didn’t die as a commander, but as someone who only cared for himself," Israeli Minister of Defense Yoav Gallant said in a statement. "This is a clear message to all of our enemies – the IDF will reach anyone who attempts to harm the citizens of Israel or our security forces, and we will bring you to justice."

The IDF initially said they were "checking the possibility" that the Hamas leader was among three militants killed in an operation in Gaza and were working to confirm identification through dental images and DNA testing.

Katz said Sinwar's death "creates a possibility for the immediate release of the abductees and to bring about a change that will lead to a new reality in Gaza -- without Hamas and without Iranian control."

Israeli President Isaac Herzog also said in a statement that Israel "must act in every way possible to bring back the 101 hostages" still in Gaza.

Sinwar, 62, had served as Hamas' leader in Gaza since 2017 and assumed leadership of the group's political bureau after the assassination of Ismail Haniyeh in Iran this July.

President Joe Biden had been briefed on Israel’s investigation into whether Israel killed Sinwar, according to a senior administration official.

The Israelis also notified U.S. Department of Defense officials, including Secretary Lloyd Austin, about Sinwar's potential death, a U.S. defense official said earlier Thursday per a pool report.

In 1989, an Israeli court sentenced Sinwar to four life sentences for his role in killing suspected Palestinian informers and plotting to murder two Israeli soldiers.

Sinwar spent the following 22 years in prison before becoming one of more than 1,000 Palestinian detainees released in 2011 in exchange for Israeli soldier Gilad Shalit, who had been held hostage by Hamas for five years.

This is a developing story. Please check back for updates.

ABC News' Guy Davies contributed to this report.