CNN

[Breaking news update, published at 7:54 a.m. ET]

The death toll from Thursday’s collapse of a 12-story residential building in South Florida has risen to four, according to Rachel Johnson, spokesperson for Miami-Dade County mayor’s office.

[Original story, published at 7:35 a.m. ET]

The painstaking search for dozens of missing people continued Friday morning in the rubble of a partially collapsed residential building in South Florida as officials held out hope that others would be found under the mountain of debris.

At least one person was confirmed dead in Thursday’s collapse in the beachfront community of Surfside, a few miles north of Miami Beach, Miami-Dade County Mayor Daniella Levine Cava said.

The collapse left at least 99 people unaccounted for as of Thursday afternoon, officials said.

About 55 of the 136 units at Champlain Towers South crumbled at around 1:30 a.m. Thursday, leaving huge piles of rubble on the ground and materials dangling from what remained of the structure, officials said.

“We’re not giving up,” Surfside Mayor Charles Burkett told CNN Thursday night. “That is the one thing we’re not doing. We’re searching 24 hours a day, and we’re pulling as many people out as we can. That’s the whole goal right now. Nothing else matters.”

The cause of the collapse wasn’t immediately known.

Two people were pulled from the rubble, said Miami-Dade Fire Rescue Assistant Chief Ray Jadallah said, who did not disclose their medical conditions. Another 35 were rescued from standing portions of the building by first responders, he said.

Rescuers are using dogs as well as listening devices to search for survivors, Miami-Dade Fire Rescue District Chief Jason Richard told CNN.

As rescuers move through the rubble, they are accompanied by structural engineers who determine which spaces are safe and where additional shoring and other materials are needed to ensure the building doesn’t shift or come down onto rescuers, Richard explained.

“So as we move through the building, we constantly monitor, making sure that there’s no movement, every piece of rubble that we move, we have to take, make efforts to stabilize the building, inch by inch,” Richard said.

Engineers weigh in on the collapse

Although the cause of the partial collapse wasn’t known, some engineers who saw it on video shared their expertise with CNN on why the building fell the way it did.

Greg Batista, a structural engineer who did work on the building years ago, said concrete repair and a spalling problem can cause collapses.

Spalling can occur when part of the surface of the concrete peels, breaks or chips.

“Spalling can get to a point that, if not repaired, it can lead to eventual collapse, and I’ve been to places where there have been collapses of floors, of beams, of columns,” Batista said.

Batista noted that for that type of collapse to occur, the malfunction of one column is enough to bring a structure down.

“All it takes is one column, and everything can come down like a Jenga,” he said. “After having seen the video … you see the actual building coming down, and the actual collapse begins on one of the lower floors. So immediately, I see that something happened down there.”

Another structural engineer, Kit Miyamoto, who is California’s seismic safety commissioner, echoed Batista’s take.

“This collapse is a real classic … column failure, which means the building itself was supported by a series of pillars. If the pillars fail, everything fails. So that’s exactly what looked like that,” Miyamoto told CNN.

Batista offered a glimmer of hope for those unaccounted for, saying it’s entirely possible for more people to be rescued.

“If you go back to videos of building crumbling in the past, you’ve seen miracles of babies being pulled out of small voids either the day after or the week after. There’s certainly a possibility that this can happen here,” Batista said.

Kenneth Direktor, an attorney for the association of residents at the condo, said the building had “thorough engineering inspections over the last several months” in preparation for compliance with a 40-year certification.

The building was constructed in 1981, according to online Miami-Dade property records. Building standards were strengthened after highly destructive Hurricane Andrew in 1992.”What that tells you is…. nothing like this was foreseeable, at least it wasn’t seen by the engineers who were looking at the building from a structural perspective,” Direktor told CNN.

An engineer had already conducted inspections to determine needed repairs, but the only work that had actually commenced was on the roof, Direktor said.

Shimon Wdowinski, a professor with Florida International University’s Institute of Environment, told CNN he determined in a study last year that the Champlain Towers South condo showed signs of sinking in the 1990s.

The condo had a “subsidence” rate of about 2 millimeters a year from 1993 to 1999, according to his study, first reported by USA Today.

While Wdowinski said this sinking alone would likely not cause the condo’s collapse, he said it could be a contributing factor.

“If one part of the building moves with respect to the other, that could cause some tension and cracks,” he explained.

The professor said buildings in other areas had moved at higher rates, and he didn’t find the condo building’s movement unusual.

“What’s unusual is that today it collapsed,” he said.