By Martin Vassolo, Samantha J. Gross, Bianca Padro Ocasio and Jay Weaver, Miami Herald

Surfside, Florida - As the sixth day since the catastrophic collapse of the Champlain Towers South condo building came to an end, rescuers progressed with their painstaking search for survivors as families and friends await any shred of information about the fate of those who are still reported missing.

On Tuesday night, a long line of Miami-Dade County police cars and medical examiner vans headed toward the site on Collins Avenue.

The official death toll stands at 12 – but that number is expected to rise.

Miami-Dade County Mayor Daniella Levine Cava said the county is working to audit the list of the missing in an effort to remove duplicate names and provide more accurate data.

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"We are sifting through all this information... this is a slow and methodical process," she said.

As of Tuesday night, 149 people were still missing.

Going forward, Levine Cava said authorities would separate the number of dead from those accounted for, which were previously combined. She added that rescue crews continued working to find victims.

"They have been working nonstop as you know for six days," she said.

President Joe Biden will visit Surfside and the site of the condo collapse on Thursday.

Florida Governor Ron DeSantis also spoke about the magnitude of the tragedy and its global impact Tuesday, saying it has been "very heartbreaking."

He said the victims whose lives have been lost and those still missing are "invariably incredibly special people" who have touched others "all across the world."

Family members of those unaccounted for remained hopeful, despite the strong undercurrent of despair and futility. A couple of hundred people gathered Monday night at a beach vigil to remember the victims of the building collapse, with both relatives and strangers joining in the silence and the pain.

Cranes at the scene on Collins Avenue and 88th Street continue to move slowly but steadily to remove buckets of rubble. Apartments still standing next to the collapsed sections of the tower were numbered 2 through 11 in green spray paint to identify the levels.

Rescue crews continued digging a large trench through the rubble of the collapsed 136-unit Surfside condo tower. They used heavy equipment to create the trench, which is described to be 125 feet long, 20 feet wide and 40 feet deep. It was created for two purposes.

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One of the reasons is to let rescuers search for survivors in other parts of the pile with their dogs, cameras, sonar and infrared technology. It was also part of an effort to combat a "deep" fire that the county’s mayor, Levine Cava, described over the weekend as "hampering" search efforts.

Levine Cava said the smoke was the "biggest barrier" for the search-and-rescue mission. She said crews worked nonstop under the rubble to stop it, using infrared red technology, foam, water and other tactics to contain the fire and minimize the smoke, which had spread through the pile.

On Tuesday, Miami-Dade Fire Chief Alan Cominsky said 3 million pounds of concrete have been removed from the site of the condo collapse, but there are still barriers to progress in the search efforts. He said rescue workers are not going back into the west section of the building facing Collins Avenue that is still standing because it is too dangerous, and he also said that they cannot enter a large area under the rubble on the eastern side because of the same risk.

Still, officials said that rescuers have identified small voids or crevices in the rubble during the operation. While it does not necessarily mean survivors will be found, they said, they are still pursuing every possible option that could lead them to finding residents.

Behind some of the efforts to find survivors are at least two small unmanned devices sent from a Massachusetts-based company over the weekend to the Surfside collapse scene. They are equipped with technology that can aid in the search for humans, including thermal sensors and 360-degree camera views, that have previously helped authorities in hostage situations, the World Trade Center collapse on 9/11, mass shootings and to disable bombs.

As the grim search for survivors and the recovery of bodies continued, heightened attention turned to newly uncovered information about the structural problems in certain areas of the condo tower at 8777 Collins Ave.

An engineer’s 2018 report flagged "major structural damage" in the pool deck, entrance ramp and garage areas of the Champlain Towers South, yet the chief building official for the town of Surfside told residents the condo building was "in very good shape," according to minutes from a November 2018 board meeting obtained by the Miami Herald.

Ross Prieto, the chief building official who left the post last year, announced Tuesday that he has taken a leave of absence as Doral’s temporary building official.

An email posted on the town’s website showed that Chouela sent Prieto two reports: the "structural field survey report" by engineer Frank Morabito of Morabito Consultants detailing the building’s structural deficiencies, and a mechanical and electrical engineering report by Thomas E. Henz. P.E.

Then USA Today reported Monday that a letter sent in April from the president of the Champlain Towers South Condominium Association said that damage to the doomed building’s basement garage had "gotten significantly worse" since an inspection about two and a half years earlier and that deterioration of the building’s concrete was "accelerating."

The letter suggested that millions of dollars in needed repairs had been a subject of frustration among residents. The letter offers a glimpse into the events leading up to the building’s collapse.

"We have discussed, debated, and argued for years now, and will continue to do so for years to come as different items come into play," the letter said.

The April 9 letter was obtained by USA Today from a family member of two building residents missing. The author, Jean Wodnicki, president of the association’s board of directors, survived Thursday’s collapse, a condo association attorney said.

More recently, a commercial pool contractor who visited the condo building on June 22, just 36 hours before half of the structure unexpectedly collapsed, said he discovered water and related damage throughout basement-level garage.

"There was standing water all over the parking garage," the contractor, who asked not to be named, told the Herald. He noted cracking concrete and severely corroded rebar in the pool equipment room.

He also took photos, which he shared with the Herald.

The contractor visited the condo building last week to put together a bid for a cosmetic restoration of the pool as well as to price out new pool equipment – a small piece of the multimillion-dollar restoration project that just was getting underway at the 40-year-old building.

Based on public records, video footage of the building’s collapse, and other images of the property, several engineering experts told the Herald that they suspect the pool deck and parking garage area caved in first, which then caused the middle and oceanfront sections of the tower to crumble under their own weight.

In efforts to instill confidence in aging buildings, local governments across Miami-Dade County on Tuesday began to explore changes in regulations.

At a morning news conference, Levine Cava announced her administration planned its own set of recommendations after she calls a series of meetings with experts on condominium law, geology, building safety, and other areas of expertise tied to the questions raised by the Surfside collapse.

The meetings are being called "so my staff and I can develop a set of recommendations and changes that need to be made at all steps of the building process to assure a tragedy like this will never happen again," she said.

At an online forum with the Brickell Homeowners Association and Haber Law, the head of Miami’s building department outlined the city’s new plan to push owners of older buildings to get their structures inspected.

The city wants buildings taller than six stories that are 40 years or older to hire structural engineers to check for "visible signs of distress" on the structures – even if the buildings have already passed their recertification process.

"We want to go above and beyond what the code calls for," added Asael Marrero, director of the Miami Building Department. "The code, right now, doesn’t allow me to mandate it."