TEL AVIV—Israel said that one of the bodies that Hamas turned over Thursday wasn’t that of a mother who was kidnapped with her two young sons and who Hamas had said it would release this week with other dead hostages.

The Israeli military said that it had informed the Bibas family that the bodies of the boys , Ariel and Kfir, had been identified. But it said another body Israel received didn’t belong to their mother, Shiri Bibas, and didn’t match any other hostage.

“This is a violation of utmost severity,” the military said. It said that Hamas was required under a cease-fire deal to return four dead hostages. “We demand that Hamas return Shiri home along with all our hostages,” the military said.

A Hamas spokesman didn’t immediately respond to a request for comment.

Earlier, Hamas had displayed coffins that it said held the bodies of four Israeli hostages before a crowd of militants while handing them over to Israel, in a spectacle that included taunts and anti-Israel slogans.

The coffins that Hamas released each had a photo of a hostage on it, as well as a picture of Israeli Prime Minister Benjamin Netanyahu over which was written “The Killer.”

The coffins were placed on a stage before cheering spectators and cameras that broadcast the scenes on Arab television channels. The International Committee of the Red Cross, who received the remains and transferred them to Israeli custody, initially tried to shield them from view with large white screens.

Israeli broadcasters didn’t air the images.

A girl reacts as a convoy transports the bodies of deceased hostages, identified at the time by Palestinian militant groups as Oded Lifschitz, Shiri Bibas and her two children, Kfir and Ariel Bibas, who were kidnapped during the deadly October 7, 2023 attack by Hamas, on the day they were handed over under the terms of a ceasefire between Hamas and Israel, in Tel Aviv, Israel February 20, 2025. REUTERS/Nir Elias

Amid the crowds were some Palestinians recently released by Israel in the cease-fire deal who, according to the terms of the agreement, were supposed to be exiled from Gaza. Those spotted in the crowds included Mohammed Abu Warda, who was sentenced for planning a 1996 bus bombing that killed more than 40 Israelis.

For the first time since the start of a fragile cease-fire between Israel and Hamas, under which the militant group agreed to free hostages in exchange for Palestinian prisoners, those returning to Israel are dead.

Israelis clung to hope for more than a year that Shiri Bibas and her sons, who were 4 years old and 9 months old when they were kidnapped, might somehow still be alive. The family’s capture became symbolic of the horrors of the Hamas-led attack on Oct. 7, 2023, which sparked the war in Gaza.

Behind a stage set up in the southern Gazan city of Khan Younis, a banner showed Netanyahu likened to a vampire, with blood dripping down his chin, blaming him for their deaths. In the early days of the war, Hamas said Shiri Bibas and the children had been killed by an Israeli airstrike. Israel never officially confirmed their deaths or addressed the possibility that they were killed by an airstrike.

In its statement, the Israeli military said that an assessment had determined that Ariel and Kfir Bibas were killed in captivity in November 2023.

The family of Oded Lifshitz, who was 83 years old at the time of his capture, confirmed that his body was identified among those returned. Lifshitz died in captivity more than a year ago, according to the director of Israel’s National Center of Forensic Medicine.

He and his wife, Yocheved,  both peace activists , were taken from the Nir Oz kibbutz near the Gaza border. His wife, who was 85 when taken, was released alive by Hamas in October 2023 after 17 days in captivity, along with three other women. Her family says she was released because she was sick and the militants were worried the illness would spread to others in the tunnels.

“It isn’t over today,” Izhar Lifshitz, Oded Lifshitz’s son, told Army Radio on Thursday morning shortly before the handover. “It will be over when everyone is back.”

In exchange for the bodies, Israel will release a group of Palestinian prisoners. Hamas has agreed to free another six living hostages on Saturday as the first phase of the cease-fire nears its end.

An Israeli military rabbi held a short ceremony after the bodies were handed over, reading the Kaddish, a traditional Jewish prayer said over the dead, and chapters from Psalms, per requests from the families. Troops fired ceremonial shots into the air, and a convoy took the bodies across the border.

The makeshift Hostage Square in Tel Aviv, where people have gathered each week to watch captives being freed, was filled with Israeli flags on Thursday that fluttered in the early-morning rain. Once the weather cleared, a small crowd of families, friends and supporters cried and embraced each other in silence as they watched a tribute video to the hostages on a large screen set up in the center of the square.

“I’m happy to be able to put up a flag and show families here that we are with them, and they are not being forgotten,” said Gili Marcovich, a 23-year-old student who volunteered at the gathering on Thursday.

A drone view shows Palestinians and militants gathering around Red Cross vehicles on the day Hamas hands over the bodies of deceased hostages, identified at the time by Palestinian militant groups as Oded Lifschitz, Shiri Bibas and her two children Kfir and Ariel Bibas, seized during the deadly October 7, 2023 attack, as part of a ceasefire and hostages-prisoners swap deal between Hamas and Israel, in Khan Younis in the southern Gaza Strip, February 20, 2025. REUTERS/Stringer TPX IMAGES OF THE DAY

Adi Koren, 18, came with a friend whose uncle, an Israeli soldier, died in the conflict. “I just feel sadness and frustration,” Koren said.

The return of the Bibas family is especially sensitive for Israelis, many of whom have been reluctant to accept that they had died. Their fate became hard to deny in recent weeks, when Shiri Bibas wasn’t freed along with other women in the early stages of the deal.

The Bibas family lived just over a mile away from the Gaza border in the Nir Oz kibbutz. It was one of the hardest-hit communities during the attack, with locals saying a quarter of its 400 residents were killed or kidnapped by Hamas militants who overtook the town on Oct. 7. Residents said help didn’t arrive until the afternoon.

Shiri Bibas, then 33 years old, was hiding with her husband, Yarden, and their sons in a shelter in their home as the militants arrived. As they closed in, Yarden left the shelter to try to protect them, but he himself was captured. Yarden, who was held separately from his family, was released in an earlier hostage exchange under the cease-fire.

The “heart of the entire nation is torn. My own heart is torn. So is yours. And all of the world’s heart should be torn, because this demonstrates who we are dealing with, what we are dealing with such monsters,” Netanyahu said late Wednesday.

“We are grieving, we are in pain, but we are also determined to ensure that such a thing never happens again,” he said.

Write to Feliz Solomon at feliz.solomon@wsj.com and Anat Peled at anat.peled@wsj.com