Rescuers press on for a third day as families of the missing wait in anguish.

As the sun rose on Saturday, rescue workers entered the third day of an increasingly desperate search for anyone who might still be alive within the giant pile of broken concrete and twisted metal that once was a condominium building north of Miami Beach.

Four people have been confirmed dead, but the fate of 159 others remained unknown, and their family members clung to thinning threads of hope.

Not a single survivor was found on Friday or early Saturday in the smoking debris of the Champlain Towers South condominium in Surfside, Fla., and the families of the missing wrestled with growing dread. At least seven children were among those still unaccounted for.

“We know there are people in that pile,” the mayor of Surfside, Charles W. Burnett, said in a televised interview on Saturday morning. “We are going to get them out. We are going to pull them out as fast as possible.”

For family members, the wait for news has been excruciating.

“I don’t know what to think,” said Sergio Barth, whose brother Luis Barth was believed to have been in the building with his wife and 14-year-old daughter when it collapsed shortly after midnight on Thursday. “I mean, there is, I think, resignation here, with just a little, little, little esperanza — you know, hope. Miracles happen. So until the authorities say the investigation is closed, we have to keep thinking positive.”

Rachel Spiegel had kept vigil near the rubble pile, eyes fixed on the area where Unit 603, the sixth-floor condo belonging to her mother, Judy Spiegel, 66, had likely crashed to earth. “I know where my mom is,” she said.

While the cause of the collapse remained unknown, the town released documents late on Friday showing that an engineering consultant warned the building’s owners two years ago about “major structural damage” to the concrete slab below the pool deck, as well as extensive cracking and crumbling of the columns, beams and walls of the parking garage under the 13-story structure.

The engineer, Frank Morabito, did not say the building was in danger of collapse, but he did say repairs were needed to ensure its “structural integrity.”

After two initial rescues, only bodies have been found. The first victim to be identified was Stacie Fang, 54, whose teenage son, Jonah Handler, was pulled safely from the rubble in the early hours of the disaster. Family members of the missing were asked to provide DNA swabs in case they were needed to identify remains.

All day and into the night on Friday, rescuers worked through thunderstorms that left pockets of flooding, and grappled with on-and-off fires whose smoke hung over the unstable pile of rubble. Dousing them was out of the question, so emergency workers tore through the debris to get to the source of the flames and pull it away.

Search-and-rescue teams burrowed underneath the debris from a parking garage, drilling through concrete and inserting probes with cameras to peer through the rubble. Specialized hearing devices alerted them to sounds that might mean people were still alive and trapped — tapping scratching, falling debris and twisting metal.

Above, two cranes gingerly removed debris from the pile on Friday with metal claws. When they paused, firefighters with red buckets clambered up to dig by hand.

The rescue workers also used dogs and sonar equipment, but the debris pile was unstable and it was slow going, officials said. “They are in the tunnels. They’re in the water. They’re on top of the rubble pile,” Mayor Daniella Levine Cava of Miami-Dade County told CNN on Friday night.

A team of federal investigators from the National Institute of Standards and Technology was dispatched to the disaster site on Friday to begin piecing together causes of the collapse.

The investigators will look for corroded components, an undermined foundation or defects in the construction or design, engineering and architectural experts said.

The building — a 13-story structure at 8777 Collins Avenue — was about to undergo extensive repairs for corrosion and concrete spalling, or flaking, as part of a required rehabilitation for buildings when they reach 40 years of age. It is also on a plot where the land is sinking in ways neighboring properties are not.