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effort and that these families should keep hope. >> search and rescue. thank you very much, boris. i appreciate that. engineers are trying to figure out exactly what caused the surfside condo to collapse. it may take months to get the answer. but evidence of structural damage already piling up. here's cnn's drew griffin. >> reporter: as new evidence emerges of past inspections, cracks and potential danger, this short surveillance camera video itself remains the best clue so far as to how and why the champlain tower south fell in what forensic engineer calls a clean collapse. >> there was a pancake effect, so it was almost symmetric and vertical. what that causes is the structure to come straight down instead of collapsing sideways or collapsing in any other trajectory, which would mean whatever caused it, which is unknown at this point, would have caused the structure to have a clean vertical collapse of those towers.