As Israeli forces advanced on the southern Gaza city of Rafah before dawn last Sunday, an ambulance crew set out to evacuate civilians wounded by Israeli shelling. But the ambulance and its crew were hit on the way.

Several more ambulances and a fire truck headed to the scene over the next few hours to rescue them, according to the Palestine Red Crescent Society, as did a U.N. vehicle, the United Nations said. Seventeen people were dispatched in total.

Then they all went silent.

It took five days for the United Nations and Red Crescent to negotiate with the Israeli military for safe passage to search for the missing people. After receiving clearance, U.N. officials said, the retrieval team found 15 dead, most of their bodies dumped in a mass grave.

On Sunday, the United Nations said Israel had killed them — a rare accusation by the organization, which is typically cautious about assigning clear blame.

“They were killed by Israeli forces while trying to save lives,” the U.N. humanitarian chief, Tom Fletcher, said on X. “We demand answers & justice.”

The Red Crescent, the International Committee of the Red Cross and the United Nations said all of those killed were humanitarian workers who should never have come under attack. The Red Crescent called the killings a war crime and demanded accountability.

An Israeli military spokesman, Lt. Col. Nadav Shoshani, said on X on Monday that nine of those killed were Palestinian militants. He said Israeli forces “did not randomly attack” an ambulance, but that several vehicles “were identified advancing suspiciously” without headlights or emergency signals toward Israeli troops, prompting them to shoot.

U.N. officials said the vehicles were clearly marked as rescue vehicles.

Colonel Shoshani said that during the attack, Israeli forces killed a Hamas military operative, Mohammad Amin Ibrahim Shubaki, who participated in the Oct. 7, 2023 assault on Israel that sparked the war in Gaza.

He said Israeli forces had also killed eight other operatives from Hamas and Palestinian Islamic Jihad, another militant group. He accused militants of “once again exploiting medical facilities and equipment for their activities.”

He did not directly say whether the militants were in the emergency vehicles or address the identities of the other six people killed.

After firing on the vehicles, U.N. officials said, Israeli forces bulldozed and crushed the ambulances, a fire truck and the U.N. vehicle.

The Red Crescent said one medic was still missing. The lone survivor, a Red Crescent worker, was detained, beaten and released by Israeli forces the same day, according to the aid group. He told colleagues that Israeli forces had killed both of the other crew members in his ambulance, the Red Crescent and U.N. officials said.

Of the 17 people involved, 10 were Red Crescent workers, six were emergency responders from Gaza’s civil defense and one was a U.N. worker, U.N. officials said.

The top U.N. humanitarian official in Gaza, Jonathan Whittall, joined the retrieval team and posted photos on X showing the crumpled vehicles — husks of mangled metal jutting from the sand. The large navy-blue “N” on the U.N. vehicle was still visible on it.

“One by one, they were hit, they were struck. Their bodies were gathered and buried in this mass grave,” he said in a video message shared by the United Nations.

Days after they went missing, the U.N. team looking for them witnessed new scenes of chaos and violence in Rafah, including “hundreds of civilians fleeing under gunfire,” Mr. Whittall said on X. One woman was shot in the back of the head, he said.

He posted a video showing what he said came next: Two men walked toward the road, apparently to retrieve the woman’s body. Then one of them was shot, too. Mr. Whittall did not say who fired the shots.

On Thursday, the U.N. convoy found the crumpled vehicles, U.N. officials said. Hours of digging yielded one body, a civil defense worker buried beneath his firetruck, Mr. Whittall said. They returned for the remaining bodies on Sunday.

Mr. Whittall narrated the search for the bodies in the video message.

“We’re digging them out in their uniforms, with their gloves on,” he said. “They were here to save lives. Instead, they ended up in a mass grave.”

The grave, he said, was marked with the emergency light from one of the destroyed ambulances.

Colonel Shoshani, the Israeli military spokesman, said the vehicles, unlike others along the same route earlier that day, had not received permission from Israeli forces to be there.

Nebal Farsakh, a Palestine Red Crescent Society spokeswoman, said that when the ambulances set out around 3:30 a.m. on March 23, Israeli forces had not yet closed off the area as a “red zone,” where ambulances must clear their movements with Israel.

Israel did not immediately address the accusations of burying people in mass graves or crushing their vehicles.

In all, the Red Crescent said, 27 of its medics have been killed since the war began.

A cease-fire paused fighting in Gaza from January until March 18, when Israel broke it.

Rawan Sheikh Ahmad and Aaron Boxerman contributed reporting from Jerusalem.