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.

Mr. Lifshitz, a retired journalist, was captured along with his wife, Yocheved Lifshitz, who was released weeks into the war for what Hamas called “humanitarian and health reasons.”

She has described abuse and harrowing conditions in Hamas’s underground tunnels, warning that other hostages would not be able to endure them.

Before the war, Mr. Lifshitz volunteered to drive Gazans seeking medical treatment to hospitals in Israel and was a founding member of a branch of Peace Now, a group advocating a two-state solution to the Israeli-Palestinian conflict. Mr. Dekel-Chen, who was friends with Mr. Lifshitz for decades, said he “was a man truly committed to his values.”

Thousands of Israelis paid tribute to Mr. Lifshitz and the other hostages during a Thursday night rally in what has become known as Hostage Square in Tel Aviv. They were also there to pressure the Israeli government to secure the release of those still being held.

Rally speakers demanded that the Netanyahu government not let the cease-fire fall apart. The first phase of the agreement between Israel and Hamas took effect in January and is set to end in less than two weeks. Negotiations on the second phase have been delayed, leaving the fates of dozens of captives up in the air.

The fragile truce has led to the release of hostages from Gaza in exchange for Palestinians held in Israeli jails — but there are concerns among the relatives of hostages that there may not be another round of releases.