(NewsNation) — A group representing the families of Israeli hostages says 24 bodies of hostages are still being withheld in violation of a ceasefire deal.

The Hostages and Missing Families Forum issued a statement Monday morning as the release of living hostages was set to begin.

The group said only four of the 28 bodies were expected to be returned Monday.

The families were reportedly told that locating all of the remains would be difficult.

“Hamas is required to uphold the agreement and take the necessary steps to return all the deceased hostages,” Israel said in a statement acknowledging the development on Monday.

It was not clear how all of the hostages died. Some were reported to have died in the initial Oct. 7 attacks and their bodies taken hostage; others died in captivity. Some reportedly died of injuries from the initial attack, and others were killed, either by Hamas or as a result of damage from Israeli military strikes.

“This represents a blatant breach of the agreement by Hamas. We expect Israel’s government and the mediators to take immediate action to rectify this grave injustice,” the group said in a statement.

The return of hostages is a critical part of a 20-point peace plan proposed by President Donald Trump. Phase 1 of the plan includes the return of hostages and the withdrawal of some Israeli forces from Gaza. Israel is also expected to release imprisoned Palestinians.

Later steps in the plan include the total withdrawal of Israeli troops, the use of a multinational security force to oversee the deal to handle security in place of those troops, the disarmament of Hamas and a transitional government made up of qualified Palestinians and international leaders, including Trump himself.

Hamas has yet to accept proposals for the next phases of the plan, which are still in negotiation.

Any perceived violation of the plan by either side could cause the deal to collapse.

“We will not abandon any hostage. The mediators must enforce the agreement’s terms and ensure Hamas pays a price for this violation,” the statement said.