Questions mounted Monday about how a residential building in the Miami area could have collapsed so quickly and violently last week, killing at least nine people with more than 150 still unaccounted for, as desperate families feared the worst.

Experts are looking at possible pre-existing critical flaws in the structure of the apartment tower in Surfside, near Miami Beach, which pancaked into a pile of smoking rubble in the early hours of Thursday.

Rescue teams from Florida and abroad scoured the debris for signs of life in torrid heat and high humidity, but as the hours dragged on, the outlook grew increasingly grim.

An October 2018 report released by city officials late Friday revealed fears of "major structural damage" in the Champlain Towers South complex, from the concrete slab under the pool deck to the columns and beams in the parking garage.

"Though some of this damage is minor, most of the concrete deterioration needs to be repaired in a timely fashion," the consultant, Frank Morabito, wrote.

Repairs had been set to begin soon in the 40-year-old building -- but did not come soon enough.

Experts who have reviewed video footage of the collapse are now looking at the idea of "progressive collapse" -- when an initial failure, perhaps in the parking garage or even below, would have snowballed into tragedy.

"It does appear to start either at or very near the bottom of the structure," Donald Dusenberry, a consulting engineer who has investigated many structural collapses, told The New York Times.

"It's not like there's a failure high and it pancaked down."

'Forensic value'

On Sunday, more than 100 people visited the site in three groups -- morning, afternoon and evening -- escorted by Miami area police and fire department personnel.

Florida Governor Ron DeSantis said debris with "forensic value" was being taken to a large warehouse to be inspected as investigators seek to determine what happened.

"As of now, the number of confirmed fatalities remains at nine," Miami-Dade County Mayor Daniella Levine Cava told reporters on Sunday.

Israeli and Mexican engineers and search-and-rescue specialists have joined an army of American workers at the site.

Six to eight rescue squads, backed by two huge cranes and aided by sniffer dogs, are "on the pile actually searching at any given time," she said.

Rescuers have dug an enormous trench -- 125 feet long, 20 feet wide and 40 feet deep (38 by six by 12 meters) -- through the mountain of debris, she added.

Surfside Mayor Charles Burkett said accommodation was being found for anyone wishing to evacuate the tower's nearly identical "sister" building a block away, though no structural problems have yet been identified there.

"We don't have a resource problem," Burkett told ABC's "This Week." "We have a luck problem. We need to get more lucky right now."

International effort

The 12-story oceanfront Champlain Towers South pancaked as residents slept. Surveillance video of the collapse showed it coming down in just a few seconds.

Jake Samuelson's grandparents, Arnie and Myriam Notkin, are among those missing, although his mother continues to receive phone calls from their landline, with only static on the other end, the family told local media.

"We are trying to rationalize what is happening here, we are trying to get answers," Samuelson told Local 10 news, unaware whether the calls were from his grandparents or mechanical failure.

President Joe Biden said Sunday his administration would coordinate with local officials and was "ready to provide any support or assistance that is needed."

"This is an unimaginably difficult time for the families enduring this tragedy," he said in a statement. "My heart goes out to every single person suffering during this awful moment."

'Make every effort'

Many members of the local Jewish community were among those affected by the tragedy, and Israel had vowed to help with the agonizing search.

About half of Surfside's population is Jewish, including many members of the Hasidic Chabad-Lubavitch movement, according to the Israeli media.

So far, four victims have been identified by local police: Gladys and Antonio Lozano, respectively aged 79 and 83; 54-year-old Stacie Fang, whose 15-year-old son was rescued from the debris Thursday; and Manuel LaFont, also 54.

At least 18 Latin American nationals are among the missing -- including Uruguayans, Argentines and Paraguayans. Canada has also said at least four of its citizens may be "affected," without elaborating.