It was shortly after noon Tuesday when Intisar Sultan walked through the clusters of dirty children and fatigued adults, leaving behind a U.N. school that had been turned into a refuge for families hoping to escape the fighting around them.

She walked out its doors without her son, Abdullah, 19, who had died along with two cousins hours earlier in an Israeli airstrike that hit the school in Gaza City. They had been returning to bed from the bathroom.

"We left our house for this shelter away from the fire, away from the shelling. But they followed us here," she cried uncontrollably. "This place was supposed to be safe."

In the sliver of land called the Gaza Strip, thousands of Palestinians are trapped between advancing Israeli forces and Hamas gunmen, who often fight from the houses, high-rise apartment buildings and office compounds of the crowded neighborhoods.

The conflict on the ground has made the desperate search for refuge more difficult for Palestinian civilians. By Tuesday, more than 15,000 Palestinians had arrived in the nearly two dozen U.N. emergency shelters, some entering rooms that days earlier had hosted classes.

"People are terrorized by this situation, and they have a right to be," said John Ging, head of the U.N. Relief and Works Agency in Gaza. "They are fleeing their homes thinking they are going into a U.N. location, that they will be safe. But there is nowhere to flee. They are trapped."

Less than 17 hours after Abdullah and his cousins died, Israeli artillery hit another U.N. shelter located in a school inside the Jabalya refugee camp. At least 40 people died in the strike.

Israel said Hamas fighters had fired mortars from the school and named two of the dead as heads of Hamas mortar units. Israel also released a video showing Hamas fighters launching rockets and mortars from another school in 2006.

U.N. officials called for a probe of the attacks on both schools. "For a long time, the school was filling up -- women, children and others fleeing the conflict. For a military that apparently has very good surveillance, it would have been clear that these were people fleeing the conflict," said Christopher Gunness, a U.N. spokesman, adding that officials of the world body had given the Israeli military satellite coordinates of all their schools. "All our facilities are clearly marked. We want an impartial investigation to find out what happened."

Many people in Gaza are making painful choices about whether to remain in their homes or flee.

Amenhe al-Douse, 55, said that 70 people had crowded into her family's apartment building and that they were too scared to leave. "I am afraid if I go outside I'll be shot," she said, speaking by phone. "We are living without electricity. There's no water."

Her son, Eyad, an auto mechanic living in Ramallah in the West Bank, said travel restrictions have kept him from seeing his parents for more than a decade. Now, from afar, he worries about them day and night.

"I am so anxious," he said. "What if they are hiding something from me? What if everyone over there is dead?"

On Saturday, Israeli jets dropped leaflets ordering the residents in northern Gaza to leave or risk death, said Samir Sultan, Intisar Sultan's brother. The Sultans decided to leave their house in Beit Hanoun.

They went to stay with their large extended family near the Jabalya refugee camp. By Sunday, that area was under assault. "The shelling attacked our family houses. Six houses were fully destroyed, and 15 were partially destroyed. So we all fled," Samir Sultan said.

On Monday, the group arrived at the Asma bint Bakr elementary school in the Shati refugee camp in Gaza City. There were 11 families. They had left swiftly, with barely their possessions. U.N. workers registered them and dispatched them to a classroom.

Hamada Sultan, 21, recalled seeing his brother Abdullah, along with his cousins Rawi, 25, and Hussein, 24, leave the classroom and head to the toilets, which were in another building. A few minutes later, he heard the explosions.

"I never imagined it would be my brother. I never thought they would target the shelter," Hamada said. "When the ambulance came, I searched for my brother. I never found him. His body was in pieces."

On Tuesday, the smell of blood floated in the air, witnesses said. Around the debris of the toilet, blood and shrapnel peppered the remaining walls. Shoes, tattered shirts and pieces of flesh specked the ground.

The Sultans fled again. Samir took Intisar out of the compound and headed to another U.N. school.

By the time they reached the next school, in the same neighborhood, all their other relatives had arrived. They sat in a classroom, on benches and on the floor, offering condolences to Intisar. They appeared tired and hungry. Outside, women were doing laundry. One family of four had one blanket at a time when nights are cold.

Rafeek Sultan said he left his elderly mother and father behind. They were too feeble, so they chose to stay at home. He wondered whether they had made the correct decision and said he felt guilty. "I don't know what will happen to them," he added.

At the school, there was no food, no milk, no diapers, nothing for basic needs, he said, as his three small children sat beside him. "We thank UNRWA for helping us, but we can't survive like this," he said, referring to the U.N. agency that provides aid to the Palestinians.

Hamada worried about his mother, Intisar, seated inside the classroom, lost in thought. "Our house is gone. My brother is gone. My future, as well," he said.

No one thinks this will be their last place of refuge.

"We're not safe anymore. We know that," Samir said. "But what else can I do? I have no other option. At least in this shelter, I won't smell blood."