Investigators were working Thursday to determine what caused a 40-year-old South Florida beachside condo building to collapse, killing at least one person and injuring 11 others. Rescuers continued to search for survivors after part of the 12-story condominium pancaked shortly before 2 a.m. About half of the building's more than 130 units were affected. As of Thursday evening, almost 100 people remained unaccounted for. A researcher at Florida International University said the building was constructed on reclaimed wetlands and was determined to be unstable a year ago. There also was ongoing roofing work being done on the building.