Fire and smoke coming from deep inside the concrete and metal remains of a collapsed 12-story condominium tower near Miami hampered rescue efforts on Saturday (local time) as emergency workers raced to recover any survivors beneath the mountain of rubble.
Rescuers used infrared technology, water and foam to battle the blaze, whose source was unclear.
Miami-Dade Mayor Daniella Levine Cava said the smoke has been the biggest challenge. In a news conference, she described the blaze as "very deep" and said rescuers faced "incredible difficulties" because of the flames.
A fire hose blasted one of the lower floors on the north side of the tower as white smoke or steam streamed out, and a bitter, sulphur-like smell hung in the air.
"The stench is very thick," Florida Governor Ron DeSantis said.
A crane removed pieces of debris from the more than 30-foot pile in the city of Surfside, and scores of rescuers used big machines, small buckets, drones, microphones and their own hands to pick through the rubble.
Among those anxiously awaiting word of missing loved ones was Rachel Spiegel, whose mother, 66-year-old Judy Spiegel, lived on the sixth floor. Speaking alongside her siblings, she said on Saturday "we're trying to hold it together".
"I know my mom is a fighter. I know she loves us. I know she doesn't want to give up. So, you know, it's day three, so it's hard," Spiegel said.
One hundred and fifty-nine people were still unaccounted for two days after Thursday's collapse, which killed at least four.
Authorities announced they were beginning an audit of buildings nearing their 40-year review — like the fallen Champlain Towers South — to make sure they're safe. The mayor asked other cities in the county to join the building review and said there would be state and federal funding to help.
Federal Emergency Management Agency officials had joined local and state authorities at the site, DeSantis said. He added that a nearby "sister building" of the collapsed tower was also being looked at because it was built at the same time and by the same designer.
Surfside Mayor Charles Burkett said he was working on a plan to temporarily relocate residents of the Champlain Towers North, which was constructed the same year and sits about 100 yards away from the collapsed building, and that FEMA had agreed to pay for lodging.
Burkett added he was also trying to arrange an emergency inspection and until that happened, he couldn't tell residents whether they were safe in their homes.
"I know that the identical building collapsed for an inexplicable reason," Burkett said. "Buildings in the United States do not just fall down. ... Something very, very wrong was going on at that building, and we need to find out."
The mayor said he didn't plan to order residents to evacuate, but if he lived there: "I'd be gone."
Surfside city staffers were also gathering details about a third building, Champlain Towers East, which was built in a different style and appears to have been constructed at a different time.
The news came after word of a 2018 engineering report that showed the building had "major structural damage" to a concrete slab below its pool deck that needed extensive repairs, part of a series of documents released by the city of Surfside.
While officials said no cause for the collapse had been determined, DeSantis said a "definitive answer" was needed in a timely manner. Video showed the centre of the building appearing to tumble down first, followed by a section nearer to the beach.
The 2018 report was part of preliminary work by the engineering company conducting the building's required inspections for a recertification due this year of the building's structural integrity at 40 years. The condominium tower was built in 1981.
The government of Israel said it was sending a team of engineering and rescue specialists to assist in the search. Israeli media have reported some 20 citizens of that country were believed to be among the missing.
Another 22 people unaccounted for were from Argentina, Venezuela, Uruguay and Paraguay, including relatives of Paraguayan first lady Silvana de Abdo Benítez.