The rescue teams at the collapsed office tower in Bangkok thought they’d found a miracle on Sunday night: a channel into the basement, leading to a wide-open space where workers still unaccounted for might have lived through Friday’s giant earthquake.

“We thought for sure we’d find someone,” said Piyalux Thinkaew, chief of operations for the Ruamkatanyu Foundation, one of Thailand’s leading emergency organizations. “It was a whole room. It was big.”

Around a dozen rescuers stepped in. They were from Thailand, China, the United States and Israel. They could see the foundation’s pillars holding strong. But the room was empty.

The next morning, another wisp of hope arrived when infrared sensors found potential signs of life. The recovery work fell silent. But after a while, there was nothing. No one was pulled out alive or dead on Monday by the time the clock ticked past the so-called golden window — the 72 hours in which survival is most likely.

“Hope is dimming,” Mr. Piyalux said. “We’re very disappointed.”

Was rescue becoming recovery? Not officially. Around 80 workers were still under the pile of chest-crushing rubble and steel that was tall enough to be seen from blocks away. Rescue workers said they were racing against time, as if 72 hours really meant 96, or maybe more.