▶ Watch Video: Audio released of first responders at deadly condo collapse in Florida The confirmed death toll in the condo building collapse in Surfside, Florida, rose to at least 12 on Tuesday, according to Miami-Dade County Mayor Daniella Levine Cava. Search and rescue crews have removed more than 3 million pounds of debris from the collapse site as they search for the remaining 149 people who are unaccounted for. First responders expressed their shock in some of the first calls to come from the scene on the morning of the collapse. “We have a 13-story building with most of the building gone. This is gonna be high priority,” one responder said. “This building does not look stable,” per the audio recording. “I see many people on the balconies,” a responder said. “The building is gone. There’s no elevator. This is nothing. I mean, it almost resembles the Trade Center.” Evacuating residents said it “sounded like they heard a bomb,” according to a responder. What we know about the victims of the Florida condo collapse It’s not yet clear how the building collapsed in the middle of night, though it’s since come to the light that it had structural damage caused by failed waterproofing under the deck. A letter from the condo board’s president to residents in April said repairs would cost millions. “It could have been a number of things that alone wouldn’t have caused the problem, but together with a trigger caused the problem,” said structural engineer Allyn Kilsheimer, who was hired by the city of Surfside to investigate the collapse. Klisheimer has extensive experience investigating other disasters like the 9/11 terrorist attack on the Pentagon and the Florida International University bridge collapse in 2018. He said one piece of the puzzle is what a lobby attendant heard before the main collapse. “He heard a muffled boom, and then X number of seconds later, a much bigger boom, and then Y seconds after that, a much bigger boom. So there were three booms. I think I understand the second one, I think I understand the third one. I don’t understand the first one,” Kilsheimer said. Two months ago, condo board president Jean Wodnicki wrote a letter to building residents because the price tag to make repairs that were highlighted in a 2018 engineering report grew to $15 million. The survey had found “major structural damage” to the concrete slab. “The observable damage such as in the garage has gotten significantly worse adding the concrete deterioration is accelerating”, Wodnicki wrote. “The way I read those documents there is nothing that I saw that if I would have read them at the point in time would have made me think the building is going to come down,” Kilsheimer told CBS News. Photos: Florida beachfront condo collapses 28 photos Sarah Lynch Baldwin and Michael Kaplan contributed to this report.