With the senior leadership of Hamas shattered by a recent series of assassinations allegedly carried out by Israel, Yahya Sinwar, one of the key architects of the Oct. 7 Hamas attacks on Israel, now appears to be the de facto boss of the terrorist organization, experts said.

The 61-year-old leader of Hamas in Gaza is also among the top targets sought by Israel, which placed a $400,000 bounty on his head following the Oct. 7 surprise attack on Israel that left more than 1,200 people dead and 240 taken hostage.

"The real guy that the Israelis want to get and will likely eventually get is Sinwar and he's in a tunnel likely somewhere in Gaza, still running the show within Gaza," said ABC News contributor Stephen Ganyard, a retired Marine colonel and a former deputy assistant secretary for the U.S. State Department.

Israeli officials announced Thursday that they killed Mohammed Deif, commander of Hamas' military wing, in a "precise, targeted strike" on July 13 in the southern Gaza town of Khan Younis. Deif and Sinwar were allegedly the masterminds of the Oct. 7 attack on Israel.

"In a world where you can be anything, Mohammed Deif chose to be a mastermind of terrorism," Israel Defense Forces said in a post on X Thursday, confirming that he had been "eliminated."

News of Deif's demise came a day after Iranian officials confirmed that Hamas political leader Ismail Haniyeh was killed in a bombing at a guest house in Tehran, where he was staying while attending the inauguration of Iran's president-elect, Masoud Pezeshkian. While Israel has not claimed responsibility for Haniyeh's death, Iranian Supreme Leader Ayatollah Ali Khamenei has called for "revenge" against Israel.

IDF officials also announced that they had killed top Hezbollah military commander Fuad Shukr in a precision missile strike Tuesday in Beirut, claiming he had been orchestrating drone and rocket attacks on northern Israel, including one on Saturday in the Israeli-controlled Golan Heights that killed 12 children and teenagers playing soccer.

The assassinations of the Hamas senior leaders have apparently left Sinwar calling the shots for Hamas, Ganyard said, at a time when negotiations involving the White House have been underway for a cease-fire in Gaza and the release of the remaining Israeli hostages.

"So Sinwar is the guy," said Ganyard. "Whether one of the political operatives gets taken out, they can still do the negotiations because eventually, Sinwar is going to have to agree to whatever negotiations go on."

Ganyard said he expects the assassination of Haniyeh will put the Israel-Hamas cease-fire negotiations on hold as Iran decides how to retaliate for the death of Haniyeh on its soil.

"Who's going to eventually call the shots is Sinwar. He's the guy that's going to have to agree to any kind of peace negotiation with the Israelis," Ganyard said.

Who is Yahya Sinwar?

Yahya Sinwar has not been publicly heard from since Oct. 7, when Hamas and affiliated groups launched the surprise attack in Israel.

Sinwar helped establish Hamas in the late 1980s. In 1989, an Israeli court sentenced him to four life sentences for his role in killing suspected Palestinian informers and plotting to murder two Israeli soldiers. He spent 22 years in prison and was one of more than 1,000 Palestinian detainees who were released in 2011 in exchange for Israeli soldier Gilad Shalit, who had been held hostage by Hamas for five years.

At the time of his imprisonment, Sinwar was head of Hamas' infamous internal security arm, Al-Majd. Israeli and Palestinian sources said that his job was to investigate members of Hamas who were potentially working with the Israelis.

In an interview in December, Michael Koubi, a former officer in Shin Bet, Israel's internal security organization, said he interrogated Sinwar, while he was a prisoner, for more than 150 hours.

Koubi described Sinwar as "tough," devoid of emotions but "not a psychopath."

According to ABC News , Koubi said that Sinwar – dubbed "the butcher of Khan Younis," for the town in Gaza that he is from – boasted during his interrogations about killing suspected Palestinian informants with "a razor blade" and "a machete."

In 2017, six years after his release from an Israeli prison, Sinwar was elected the overall chief of Hamas in the Gaza Strip.

Sinwar's ideology and long-term hatred toward Israel were what motivated him to attack the country on Oct. 7, according to Koubi.

Following the attack on Israel, Israeli Prime Minister Benjamin Netanyahu said on Dec. 6 that it was "only a matter of time" before Sinwar is located. Israeli military leaders have described him as "a dead man walking."

Koubi said that he expects Sinwar will eventually go down fighting.

"He wants to die a hero of the slum, as a hero of Hamas, as a hero of the Gaza people," Koubi said.