The bodies of the children who died outside the United Nations school here were laid out in a long row on the ground. Some were wrapped in the vivid green flag of Hamas, some were in white shrouds, and some were in the yellow flag of Fatah, which is rarely seen these days in Hamas-run Gaza.

Hundreds of Gazans crowded around, staring at the little faces, some of them with dark eyes still open, but dulled.

Abdel Minaim Hasan, 37, knelt, weeping, next to the body of his eldest daughter, Lina, 11, who was wrapped in a Hamas flag. “From now on I am Hamas!” he cried. “I choose resistance!” But then he cursed Arab nations for ignoring the plight of the Gazans. “The Arabs are doing nothing to protect us!” he shouted.

The streets were crowded Tuesday evening when the mortar shells struck, Mr. Hasan said. “We were in a United Nations school, we were so far from the tanks.” There were many children around, and he gave Lina a shekel to run to a nearby grocery store. She was hit by shrapnel and died.

Some 280 families — 1,674 people — had been seeking shelter inside the school, Al Fakhura, according to the United Nations Relief and Works Agency, which helps Palestinian refugees and their descendants and which runs the school. Most came from farther north in Gaza, near Beit Lahiya, where the fighting has been intense, an hour’s walk away. Israeli forces ordered them to evacuate their homes for their own safety.

But Al Fakhura, set in the northern part of the densely packed Jabaliya refugee camp north of Gaza City, is in a crowded neighborhood full of Hamas fighters. Israel said that a preliminary investigation showed that mortar fire from the school compound prompted Israeli forces to return fire. The Israeli mortar rounds killed as many as 40 people outside the school; Palestinian hospital officials said Tuesday that 10 of the dead were children and 5 were women.

Residents of the neighborhood said two brothers who were Hamas fighters were in the area at the time of the attack. The military identified them as Imad Abu Asker and Hassan Abu Asker, and said they had been killed. But the residents also said the mortar fire had not come from the school compound, but from elsewhere in the neighborhood.

The director of the United Nations relief agency in Gaza, John Ging, who was not at the school when it was attacked, denied that Hamas fighters had been taking shelter in the school or using its premises. “There are no military people inside the school; it is fully controlled,” he said.

Mr. Ging put the death toll at 40 and said 15 more people were critically wounded and 40 others less seriously wounded. He called for an international investigation. “Those who died or were injured deserve accountability,” he said.

Mr. Ging spoke at the school during a three-hour lull in the fighting on Wednesday. Israel’s Foreign Ministry announced that a lull would take place every other day to allow humanitarian organizations to deliver supplies and Gazans to emerge from their houses and shelters to shop.

But while there is food, Gazans are running out of cash to buy it, with banks shut and A.T.M.’s empty and nonfunctional because of a lack of electricity.

The lull was also used for funerals, and a senior Hamas official, Mushir al-Masri, emerged from hiding to congratulate those he called martyrs. Some parents shook his hand; some stared at him coldly. On a loudspeaker, a man praised the dead and said: “What Israel is doing is bringing us unity again! We are all together!”

That idea appeared to explain the willingness of Hamas to have Fatah flags in such evidence.

Halfway along the row of bodies — uncountable in the press of the mourning crowd — Huda Deed was weeping. She lost nine members of her extended family, ages 3 to 25. “Look, they’ve lined them up like a ruler!” she said, inconsolable. But when asked for an interview by Al Aksa television, the Hamas channel, she refused.

Israel blames Hamas for civilian casualties, saying it knowingly endangers civilians by operating among them. Asked later about the presence of Hamas fighters in the streets around the school, Ms. Deed avoided a direct answer, but implied that her heart was not with Hamas. “We want to live like everyone else in the world,” she said.

Inside the school, Shaaban Hasouni, 47, said he came from Beit Lahiya on Sunday with eight members of his family, ages 2 to 29 years old. The Israelis had told them to evacuate, first in calls to their cellphones and then with leaflets and loudspeakers, he said. They were supposed to hold up white sheets as they evacuated. But he said he and his family left their home only when Israeli forces fired some sort of gas into it. “I couldn’t see, I didn’t know what it was, but we escaped,” he said.

Mr. Hasouni was able to make a quick trip home on Wednesday morning and came back with mattresses and blankets, saying that it was so cold, even with 40 people in every classroom, that his children could not stand it.

Asked about fighters, he said: “Of course we don’t want them around us. But we don’t know who they are, we don’t know their faces. And they wear normal clothes.”

Mahmoud al-Sous, 39, arrived on a donkey cart with blankets, mattresses, water and clothes. “My children were dying from the cold,” he said. “After two days I couldn’t stand it, so I risked going back to bring these things.”

Atif Suboh, 25, here with six members of his family, was not so lucky. Three of his cousins, ages 6, 8 and 12, he said, went back to Beit Lahiya on Wednesday morning to get blankets. They died from the shelling, he said. Asked how he could have let them go, he started to wail. “They didn’t tell us they were leaving,” he said.

If he had known that fighters would be around the school, he said, “I would never be there.”

Samira Shakoura, 40, lives in the neighborhood. “It was terrifying,” she said. “We hid in the house.”

She was defiant about the presence of Hamas. “Listen, I will always open my house to protect the fighters,” she said. “We have to be patient. We are dead anyway like this. And when Hamas runs in the elections, I’ll vote for them; they have Islam.”

Nearby, while funerals went on, people surged toward a shop 200 yards away. There were at least as many people at the shop as at the funerals. A shipment of flour had arrived.