As dawn broke on Thursday, Haitham Abu Ammar combed through the rubble of the school that had become a shelter to him and thousands of other displaced Gazans. For hours, he helped people piece together the limbs of the ones they loved.

“The most painful thing I have ever experienced was picking up those pieces of flesh with my hands,” said Mr. Abu Ammar, a 27-year-old construction worker. “I never thought I would have to do such a thing.”

Early on Thursday, Israeli airstrikes hit the school complex, killing dozens of people — among them at least nine militants, the Israeli military said.

Over the course of the day, corpses and mangled limbs recovered from the rubble were wrapped in blankets, stacked in truck beds and driven to Al Aqsa Martyrs Hospital, the last major medical facility still operating in central Gaza.

Israel’s military described the airstrike as painstakingly planned. Rear Adm. Daniel Hagari told reporters that Israeli forces had tracked the militants in the school-turned-shelter for three days before opening fire.

“The Israeli military and the Shin Bet found a solution to separate the terrorists from those seeking shelter,” he said.