Why did Florida building collapse?

Questions are multiplying how a residential condominium in Florida could have collapsed so quickly and violently last week, as the slow work of sifting through the rubble stretched into a sixth day on Tuesday.

The work has been deliberate and treacherous. Two additional bodies were found on Monday, raising the count of confirmed dead to 11. That leaves 150 people still unaccounted for in the community of Surfside outside Miami.

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

Battling torrid heat and high humidity, rescue teams from Florida and abroad continued to scour the pile for any sign of life. As the hours dragged on, the outlook grew increasingly grim.

The collapse of the building left layer upon layer of intertwined flotsam, frustrating efforts to reach anyone who may have survived in a pocket of space.

"Not to say that we have seen anyone down there, but we've not gotten to the very bottom," Miami-Dade Assistant Fire Chief Raide Jadallah told reporters on Monday.

Authorities on Monday insisted they are not losing hope. Friends and neighbors of the building's occupants held a vigil on a nearby beach on Monday evening, clutching white roses and sobbing as a facilitator burned incense and played a gong.

Glow sticks and dirt laid on the sand spelled out the word "hope".Down the coast at the ruins of the building, the sound of rescuers' power tools carried on through the night with no indication anyone had been found alive.

Earlier in the day, officials said investigators were at the scene to do a preliminary review to determine if a full investigation of the incident would proceed. It is an outcome which Florida Congresswoman Debbie Wasserman-Schultz, who represents the area, said seemed likely.

Structural damage

An October 2018 report released by city officials late on 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.

A commercial pool contractor who visited the building just 36 hours before the collapse told the Miami Herald of damage he saw in the basement-level garage, including cracking concrete and severely corroded rebar under the pool.

"There was standing water all over the parking garage," said the contractor, who also provided photos and asked not to be named.

Experts who reviewed video footage of the disaster are now looking at the idea of "progressive collapse "when an initial failure, perhaps in the parking garage or even below, 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'

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.

Israeli and Mexican engineers and search-and-rescue specialists have joined an army of US workers at the site, backed by two huge cranes and sniffer dogs.

The 12-story oceanfront Champlain Towers South collapsed as residents slept. Surveillance video showed it coming down in a matter of seconds.

Many members of the local Jewish community were among those affected by the tragedy since about half of Surfside's population is Jewish, Israeli media said.

More than two dozen Latin American nationals are among the missing. Canada added at least four of its citizens may be "affected", without elaborating.

Agencies via Xinhua and Heng Weili in New York contributed to this story.