But the facility in the Nuseirat refugee camp was also packed with thousands of civilians displaced by the war, according to the U.N. Relief and Works Agency, which aids Palestinian refugees. The morgue at nearby al-Aqsa Martyr’s Hospital said late Thursday that 33 people were killed in the strike, revising down an earlier toll, the Associated Press reported. The morgue said nine children, three women and 21 men were killed, according to the AP.

The Gaza Health Ministry, citing the hospital, initially said 40 people had been killed, including 14 children and nine women. It was not immediately clear what caused the discrepancy, the AP reported.

The nose cone of a GBU-39 small-diameter bomb, which is manufactured by Boeing, was visible in footage taken by an eyewitness, Emad Abu Shawish, in the aftermath of the attack. His images were verified by Storyful and independently geolocated by The Washington Post.

“We are confident that our activities were effective in limiting and reducing the harm to civilians in the area,” Lt. Col. Peter Lerner, an IDF spokesman, said at a news conference Thursday. He said that 20 to 30 Hamas and Islamic Jihad fighters were operating from a compound inside the school, and that the strike was called off twice to limit civilian casualties.

“We are pressing the government of Israel and the IDF to be completely transparent about what happened here,” State Department spokesman Matt Miller said at a briefing Thursday.

Reviewing footage of the debris, Trevor Ball, a former explosive ordnance disposal technician for the U.S. Army, said the munition’s distinctive nose cone was used to penetrate the building’s concrete structure. Rahul Udoshi, a senior analyst on the weapons team at the defense firm Janes, and N.R. Jenzen-Jones, director of Armament Research Services, both confirmed that imagery from the strike’s aftermath appeared to show the nose of a GBU-39 small-diameter bomb.

“One of its benefits is that it can penetrate concrete, penetrate buildings, and take out floors exactly like this. So essentially what you’re seeing is that solid nose cone left over after the blast,” Ball said.

But, he added, the calculations should change when you’re using a small-diameter bomb in an area where civilians are “heavily present.”

The GBU-39’s distinct shape and capabilities originate from a U.S. Air Force requirement to provide combat aircraft with a strike option to minimize collateral damage, while at the same time being compact enough to fit in the smaller weapons bays of the F-22 and F-35 fighter jets, Jenzen-Jones said.

In a report last month, the Biden administration said that it was “reasonable to assess” that Israel has violated international law while using U.S. weapons in its military campaign in Gaza. The United States is Israel’s chief weapons supplier. But, the report found, there was insufficient information to draw a firm conclusion in any specific instances, meaning U.S. military aid could continue to flow to the country.

An analysis of several thousand distinct incident reports from Gaza, published by an independent panel of experts, found instead that Israel’s military has acted with a “systematic disregard for international humanitarian law, with recurrent attacks launched despite foreseeably disproportionate harm to civilians and civilian objects.”

Israel has drawn widespread international condemnation over the mounting civilian casualties in Gaza, where more than 36,000 people have been killed since the start of the war on Oct. 7. At least 45 people were killed in a strike on a tent camp in Rafah last month, which the IDF said targeted two Hamas officials.

Weapons experts also identified the munitions in that strike as U.S.-made GBU-39s. The fragments visible in verified images from the scene showed the cage code, or five-character sequence used to identify vendors that sell weapons to the U.S. government. The “81873” designation linked the fragment to Woodward HRT, a weapons components manufacturer registered in Valencia, Calif.

Wes J. Bryant, a former targeting professional in the U.S. Air Force, and one of the authors of the report by the independent panel, said U.S. military commanders probably would not have greenlit a strike like the one Thursday.

“With vetted intelligence that a terrorist entity was running operations from that school, as the IDF claim … we would have categorized the facility as ‘dual-use,’ meaning that the facility has both a civilian and a military purpose,” he said. “This automatically places the target at the highest possible risk level for collateral damage and civilian casualties.”

Within the U.S. Air Force, Bryant added, “a formal targeting package would have been developed and pushed up for approval at the most senior levels of military command, given that the civilian casualty estimate would be extremely high. It would most likely not be approved.”

The IDF has ramped up attacks in central Gaza in recent days, saying that Hamas fighters are operating in smaller cells across the enclave. The central region is now home to a growing population of displaced Palestinians, including thousands of people who fled an Israeli offensive in Rafah in southern Gaza last month.

On Thursday, Shawish, 37, said he was asleep at home when he awoke to his house shaking. He walked outside and heard people screaming and calling for help. At the school, Shawish said, he saw bodies on the ground, people running for safety and ambulances arriving to ferry the wounded.

People were pulling dead and injured civilians from the rubble, he said. “The scene was truly frightening,” he said, adding that people were scrambling to pull dead and injured civilians from the rubble, and that baby diapers, food, blankets and mattresses were scattered among the debris.

Another camp resident, Salama Younis, 27, said that the munitions had killed many people inside the rooms they targeted, and that there were women, children and elderly people among the wounded.

“I saw people’s food and the cooking utensils they were using,” he said. “Their clothes, covers, belongings.”

At al-Aqsa hospital, aid groups said, staff struggled to deal with the influx of casualties. “Attacks on displaced Palestinians are escalating every day and hospitals across Gaza are on the brink of collapse,” said Fikr Shalltoot, Gaza director for the U.K.-based Medical Aid for Palestinians group. “At al-Aqsa hospital there are not enough operating theaters, beds or equipment to deal with the influx of patients.”

Phillipe Lazzarini, commissioner-general for the United Nations’ Palestinian refugee agency, known as UNRWA, wrote on X on Thursday that the school had been sheltering 6,000 displaced people when it was hit overnight by Israeli forces “without prior warning to the displaced.”

“Attacking, targeting or using UN buildings for military purposes are a blatant disregard of International Humanitarian law,” he wrote.

Juliette Touma, UNRWA’s communications director, said in a statement to The Post that more than 170 UNRWA buildings — mostly schools that were turned into shelters — have been hit since the war began in October, killing more than 450 people.

“We call for investigations into all violations against the United Nations including attacks on our buildings,” Touma said.

Miriam Berger, Meg Kelly, Kyle Melnick, Ellen Francis and Lior Soroka contributed to this report.