The appearance of Mr. Ruíz fueled the hope of relatives and rescuers working at the site. “We are removing debris, taking food to the collection center and helping in any way we can because this is our city and nothing will take it away from us,” said Israel Rodríguez, 32, a volunteer rescuer.

Moments later, another member of the rescue operation descended from the towering mound of rubble. He was a lieutenant in the military, and his uniform was stained from the work. With tears in his eyes, he recounted how rescuers had heard Mr. Ruíz’s screams from within the wreckage throughout the night. There had been several other people trapped near Mr. Ruíz, but all had died. “We could only get Sergio,” explained the officer, who identified himself only by his last name, Alejandre, because he was not authorized to speak on the record.

At this and other demolished buildings, a sense of impotence accompanied the anguish of relatives who stood along the periphery, praying that their loved ones would emerge alive.

“My brother is still in there,” said Cintia Escamilla, 34, as she sat in front of a collapsed office building in the Roma neighborhood, where others had also gathered to wait for news. “Nobody has told us anything yet.”

Civil protection officials at the scene said that at least 25 people had been rescued from the destroyed building and five bodies had been recovered. But they estimated that 25 more people were still inside.

“I have been waiting since yesterday to hear of my aunt and two employees,” said Jonathan Durán, 38, who worked in the building. “To hear anything, whatever it is, because the wait is killing me.”