(CNN)The death toll from Thursday's partial collapse of a South Florida residential building has risen to four, officials said Friday morning, as a painstaking search for survivors in the rubble continued.
Three bodies were found overnight from Thursday into Friday in the wreckage of Champlain Towers South in Surfside, Miami-Dade Fire Rescue Assistant Fire Chief Ray Jadallah told reporters Friday morning.
The number of people unaccounted for is now 159, Miami-Dade County Mayor Daniella Levine Cava said -- up from the figure of 99 that officials gave Thursday afternoon.
"I want to be very clear about the numbers. They are very fluid," the mayor 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.
The search for dozens of missing people continued Friday morning at the site, a few miles north of Miami Beach.
"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.
What we know about those unaccounted for
At least 30 people believed to be missing are from several Latin American countries, according to officials from the respective countries.
At least nine are from Argentina.
They included Andrés Galfrascoli, 45, his partner Fabián Nuñez, 55, and their daughter, Sofía Galfrascoli Núñez, 6, according to a friend. The group was staying at a friend's condo for vacation.
"We don't know anything, we don't have any closure and that's what hurts," friend Nicolás Fernández told CNN.
Six Paraguayans, including the sister of Paraguayan First Lady Silvana López Moreira, the brother-in-law and their three children are still unaccounted for.
The first lady's relatives live on the 10th floor of the partially collapsed building, and Paraguay's ministry of external relations has not been able to locate the family, the ministry told CNN en Español.
Officials representing Uruguay and Venezuela also reported missing citizens, and at least six Colombians resided in the building, authorities said.
More locally, the family of a mother and grandmother who were in the section of the building that collapsed haven't heard from them, the son, Pablo Rodriguez, told CNN.
"We are praying for a miracle, but at the same time trying to be as realistic about it as possible," he said. "Until we definitely know, there is hope. It's just dwindling by the minute."
Some members from the Shul of Bal Harbour synagogue are also among those unaccounted for, Rabbi Sholom Lipskar told CNN.
"This is something that transcends our capacity for understanding," Lipskar said about the collapse. "It's a reality, we accept it and we have to learn as we do in our culture of resilience to move forward."
Rabbi Eliot Pearlson, who leads Temple Menorah, told CNN's Chris Cuomo, "It's hard to explain. This doesn't happen in America. It's doesn't happen in Miami Beach. It doesn't happen in our homes. And it's very difficult to comprehend how it's possible."
Pearlson said he saw people come together in compassion following the collapse, and his temple will host an emergency prayer service on Friday.
Three generations of one family from his temple among the unaccounted, he said.
"I have to tell you, when I walked past ground zero, there was row after row after row of firefighters who are literally waiting to rush into a building that could fall at any time," he added.
CNN's Rosa Flores reported from Surfside; Aya Elamroussi reported and wrote from Atlanta. Rebekah Riess, Amanda Watts, Theresa Waldrop, Ana Zuniga, Melissa Alonso, Jamiel Lynch, Camille Furst, Abel Alvarado, Valentina Moreira, Gerardo Lemos and Radina Gigova contributed to this report.