SURFSIDE, Fla. — The search for victims of last week's partial collapse of a condominium building was halted for much of Thursday over concerns that what remained of the structure was unstable, a painful setback for the first responders and families whom President Biden spent hours trying to comfort during a day-long visit to the area.

After a week of round-the-clock digging, officials said the search-and-rescue efforts were suspended about 2 a.m. Thursday because of “structural concerns” about what remained standing of the Champlain Towers South building. Miami-Dade Mayor Daniella Levine Cava announced at a 6:30 p.m. news conference that work had resumed where it was deemed safe to do so.

Levine Cava said the pause was needed to guarantee the safety of rescue teams. The confirmed death toll stood at 18, with 145 missing. She also said officials were preparing for the “likely demolition” of the condo tower.

“The only reason for this pause is concerns about the standing structure,” Levine Cava said. “We’ve already informed the families this morning, who are waiting and waiting, about this development.”

The news added to the somberness of Biden’s visit, as most experts increasingly think the chances of finding any additional survivors in the rubble are exceptionally small. The Miami area also was shaken by the news that auditors reviewing the stock of condo buildings have ­identified serious problems that could threaten the stability of high-rises across Miami-Dade County.

Biden, accompanied by first lady Jill Biden, spent his day visiting victims’ families, meeting with firefighters and rescue teams, and attending a briefing alongside Florida Gov. Ron DeSantis (R) and Levine Cava. The president announced that the federal government would pay 100 percent of the costs incurred by the state and Miami-Dade County in response to the Surfside condo collapse.

“There’s going to be a lot of pain and suffering and even the need for psychological help in the days and months that follow, so we’re not going anywhere,” Biden said. “Tell me what you need.”

Biden spent about three hours in a private meeting with building residents and family members of those missing, an extraordinary amount of time for a president to devote to one issue in a daily schedule.

Steve Rosenthal, a resident of the part of the building that still stands, said Biden spoke to each person in the room of 200 hundred.

“He would sit down and listen,” said Rosenthal, 72. “There were people who talked for a minute and there were people who talked for 10 minutes, and no one tapped him on the shoulder and said, ‘Hey, you got to leave.’ ”

Speaking to reporters after the meeting, Biden said the families were “going through hell.”

“You know, they had basic heart-wrenching questions: ‘Will I be able to recover the body of my son or daughter, my husband, my cousin, my mom and dad? How can I have closure without being able to bury them if I don’t get the body? What do I do?’ ” Biden said during brief public remarks.

Biden said investigators are considering a range of possible causes for the 12-story building’s collapse. He said investigators and family members discussed several possible causes with him — rusted rebar, defective concrete, disturbances to the foundation during construction of a nearby building, elevated groundwater levels because of climate change.

“I don’t think there is at this point any definitive judgment as to why it collapsed, and what can be done to prevent it from happening’ ” to other buildings, Biden said.

But the president stressed throughout the day that the federal government would work closely with Florida to deal with the crisis, despite partisan differences between him and DeSantis.

At one point, Biden turned to DeSantis, a potential GOP presidential contender in 2024 and a frequent critic of Biden’s policies, and tapped him on his wrist. Biden told DeSantis he was grateful that local, state and federal officials have put aside partisanship.

“You’ve recognized the severity of this tragedy from Day One, and you’ve been very supportive,” Biden said. DeSantis responded, adding that there have been no bureaucratic barriers to getting the help Florida needed.

“I promise there will be none,” Biden added.

But at the collapse site, families and first responders say what they really continue to need is luck.

Miami-Dade Fire Chief Alan Cominsky said authorities suspended search-and-rescue efforts before dawn Thursday after three crack monitors signaled expansion in crevices in the part of the building that remains standing.

Authorities then found that there had been six to 12 inches of movement and determined that a large column hanging from the structure could fall and damage the support columns in the subterranean garage area.

Slight movement also was detected in concrete slabs in the south side of the structure near a corner of the building that could cause additional failure, according to Cominsky

No survivors have been found at the site since the first few hours after the collapse June 24.

Although officials stressed that they have no plans to abandon their search for survivors, rescue teams said their work has been hampered by the shifting debris pile and the vast amount of concrete that crumbled to the ground.

“This is the most complicated site I have seen in my life,” said Elan Edri, deputy commander of the Israeli National Rescue Unit, members of which arrived in Surfside over the weekend. “The collapse was very, very unique — the building collapsed inside itself and collapsed in four different phases, and each phase collapsed on the former one.”

On Wednesday, U.S. and Israeli rescue officials told reporters that they were buoyed by the discovery of air pockets and tunnels in the debris that possibly could maintain life.

Yet, medical professionals say they are increasingly skeptical that anyone else will be found alive.

“From a medical standpoint, to think of someone who has gone without food and water, in heat, and with injuries, it’s highly unlikely,” said Trey Todd, an orthopedic surgeon who also is the president of the Louisiana Search and Rescue K-9 team. “It is possible that maybe someone is finding water dripping as they spray the rubble . . . but the vast majority of them are going to be irreparably injured, and it doesn’t take long for infection to set in, either.”

In 2019, Todd and his team spent days searching for victims after the collapse of the under-construction Hard Rock Hotel in New Orleans.

Three construction workers were killed in that collapse, and recovering the bodies took months, Todd said. Despite his skepticism, Todd does not fault Florida officials for their reluctance to declare their mission solely a recovery operation at this point.

“We do this to maintain hope for the families until we can bring closure,” Todd said. “The more hope, the better, as long as it’s not completely unrealistic.”

Todd added that search teams in the United States do not have a lot of experience with major building collapses, the exception being the twin towers during 9/11, the bombing of the Murrah Federal Building in Oklahoma City in 1995, and the collapse of Champlain Towers South here last week.

“You have different scenarios with mudslides, earthquakes and tornadoes, but this experience is much different,” Todd said. “The debris field [of a tornado] is vast but not nearly as deep as when things are stacked 10 stories on top of each other.”

Speaking to reporters Thursday, Cominsky, the county fire chief, said rescuers did hear the voice of a woman in the wreckage during the initial hours after the collapse. The voice could be heard “for several hours,” he said, but later went silent and has not been heard since.

“Unfortunately, we didn’t have success with that,” he said.

During his meeting with first responders, Biden acknowledged that their work could take a mental toll.

“What you’re doing now is just hard as hell to deal with, even psychologically to deal with,” Biden told them. “And I just wanted to say thank you. Thank you, thank you, thank you.”

At times, Biden sought to inject humor into the gathering. He joked that there are three political parties in his home state of Delaware — Democrats, Republicans and the firefighters — and that he was a firefighter.

“All kidding aside, the job you’re doing is incredible,” he said. “You know, most of the time people don’t really fully appreciate you till they need you.”

Miami-Dade officials, meanwhile, were racing to make sure other properties in the county were not vulnerable to catastrophic failures.

Last week, a team of county auditors began reinspecting properties and records for safety compliance.

So far, auditors have identified 24 high-rise condo buildings that were flagged as “unsafe” by inspectors in recent years. Public records show that many of the cases were opened years ago and never closed.

Not all of them involve structural problems that could create a risk of collapse, but all of the structures were at one point deemed “unsafe” by building inspectors.

Problems range from serious structural weaknesses such as rusted support beams to smaller problems such as inadequate parking lot illumination.

County spokesperson Tere Florin said that the 24 high-rise condo buildings are in the middle of a 40-year review but that certification has been held up either because a report was never submitted to the county or deficiencies have not been corrected as required.

“These deficiencies are not necessarily related to the building’s structural integrity but must nevertheless be corrected to ensure safe and comfortable living conditions,” Florin said in an email.

But during an inspection conducted Monday at the Royal Oaks Condominium, a four-story residential building about five miles inland near Miami-Dade’s northern border, inspectors found a rusted steel column supporting three floors of exterior patios.

County officials placed placards marked “unsafe” on the patios in question, and residents were told not to step foot on the patios until another structural investigation can be completed.

In accordance with the county’s requirements, the condo association has contacted an outside company to help shore up this section of the building, a county spokesperson said.

Colby Itkowtiz, Lateshia Beachum and Paulina Firozi contributed to this report.