Mexican rescuers were working through the early hours Saturday in a desperate search for survivors amidst twisted rubble of collapsed buildings three days after a deadly earthquake. Across Mexico City, with exhausted emergency workers still reporting signs of life, the head of the national disaster management agency, Luis Felipe Puente, warned the coming hours would be critical. "Tonight will be tough, because a lot of time has passed (since Tuesdays quake). But we wont give up," he said. "Time has gotten the best of us. There are structures that are very complicated to access. But were going to keep fighting for the families" of those feared trapped inside, he told TV network Televisa. The 72-hour-mark expired at 1:14 pm (1814 GMT) Friday. Three days is the limit that experts say people trapped in rubble without water, often with crushed limbs, can hold on. Across the capital of 20 million people, many whose dwellings had become uninhabitable sought a place to call home, raising the specter of a housing shortage. Senior officials said there could be 20,000 badly damaged homes in the states of Morelos and Puebla. Tuesdays massive quake, which called nearly 300 people, struck on the anniversary of the deadly 1985 tremor that left 5,000 people dead in Mexico City. Mexico City recorded the highest number of fatalities in this week’s quake: 157, with more bodies certain to be found. The rest of the deaths occurred in Morelos, Mexico, Puebla, Guerrero and Oaxaca.