BOGOTÁ, Colombia — Gildardo Ramírez reached Mocoa on Sunday, only to find a scene of rubble where his Aunt Claudia’s neighborhood had been. He looked desperately through the ruins for his missing aunt, who he said had not been seen since a flood of mud and debris plowed through Mocoa the night before.

“I arrived to the house, and it was destroyed,” he said by telephone from Mocoa, a small city in the southern mountains of Colombia near the border with Ecuador. “The only thing I found was a backpack.”

Anxious relatives like Mr. Ramírez and more than 1,500 rescue workers raced on Sunday to find anyone who might still be alive and trapped in the wreckage in Mocoa, where parts of the city have been wiped off the map by a deluge of mud and floodwaters.