No deaths had yet been confirmed, and many lives may have been saved because the 54-unit apartment building was evacuated after cracks appeared on Friday, said Jaime Enrique Gomez, acting director of Medellin's disaster preparedness agency.
After the tower collapsed at about 8:40 p.m. (9:40 p.m. EDT; 0140 GMT) Saturday, five other buildings in the "Space" development were evacuated as well.
Gomez said about 150 rescuers were trying to find the missing in the ruins of the 22-story building in one of the most exclusive neighborhoods of Medellin, Colombia's second-largest city.
"We cannot lose hope of finding them alive," he said by telephone.
The company that built the development, CDO Constructora, issued a statement on Friday saying that a flaw had been found in a concrete column and that it was under repair. The company assured that "there is no danger for the rest of the building."
The mayor's office said nine of the missing were workers, apparently trying to make repairs, and the others were a watchman and a resident.
Two other workers were rescued, injured, from the ruins.
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