The Israeli military said on Friday that Hamza Ziyadne, an Arab citizen of Israel held hostage in Gaza, had been killed in the Palestinian enclave, as efforts by mediators to broker a truce between Israel and Hamas to free hostages have seen little success.

Over 15 months after the Hamas-led attack on Oct. 7, 2023 prompted the war in Gaza, around 98 hostages remain in Gaza; roughly 36 are presumed dead by the Israeli authorities.

The confirmation of Mr. Ziyadne’s death comes just a day after family and friends buried his father, 53-year-old Youssef Ziyadne, who was also taken hostage. The Israeli military said their bodies been found together in a tunnel under the southern Gaza city of Rafah alongside their dead captors. It was not clear when they were found.

Before they were found, neither hostage had been designated as presumed dead by Israeli officials, who have sought to use intelligence to assess the condition of the remaining hostages. That was likely to further escalate fears among the families of the remaining captives in Gaza that their relatives might have already suffered the same fate.

It was not immediately clear how the Ziyadnes died: Some hostages have been killed in Israeli airstrikes, while Israel has said that others were executed by their captors. Nadav Shoshani, an Israeli military spokesman, said on Wednesday — after Youssef’s death was confirmed — that the military was still investigating.

Israel’s Arab citizens, like Mr. Ziyadne, were not spared in Hamas’s attack. At least 17 were killed and several others taken hostage. Around 1,200 people were killed in the assault and 250 taken hostage, according to Israel.