For Jonathan Dekel-Chen, every day this week has been a mixture of joy and grief. He is celebrating the return of his son Sagui, who was released over the weekend as part of the cease-fire deal with Hamas. But reminders of Sagui’s ordeal, and the torments of the remaining hostages, are impossible to escape.

“Today is a day with very mixed feelings,” Mr. Dekel-Chen said in an interview on Thursday.

He had just visited his son in a Tel Aviv-area hospital on a day when Hamas turned over coffins that were said to contain the remains of four of Mr. Dekel-Chen’s neighbors in Kibbutz Nir Oz, where about a quarter of the 400 residents were either killed or taken hostage on Oct. 7, 2023.

It has been 504 days since the Hamas-led attack, and roughly 60 hostages have yet to come home. “We need to double down now on getting all the hostages home,” Mr. Dekel-Chen said. The four bodies returned on Thursday were said to include three members of the Bibas family — Ariel Bibas, 4, and Kfir Bibas, who was just 10 months old, and their mother, Shiri Bibas. The Bibases came to symbolize the plight of the captives after videos of them being taken to Gaza went viral.

But early Friday, the Israeli military announced that the remains in what was said to be Ms. Bibas’s coffin did not match the identity of any of the hostages. “This is a violation of utmost severity,” the military said.

The authorities did confirm the children’s remains, and those of Oded Lifshitz, who was 83 when he was killed in captivity by the militant group Palestinian Islamic Jihad, according to Prime Minister Benjamin Netanyahu’s office.