The fire broke out on the 22-story building's second floor about 4:15 a.m. in the 4800-block of South Lake Park Avenue.
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Two women, ages 84 and 64, were transported to University of Chicago Medical Center for smoke inhalation, fire officials said. They were in good condition.
A firefighter was also transported there for minor injuries.
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Ten people also refused to be transported, officials said.
Chicago fire officials had finished conducting a search of the Harper Square Cooperative high-rise building by about 5:30 a.m. to make sure everyone had made it out safely, and the fire had been extinguished.
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Some residents were forced out into the cold, and Chicago firefighters were working in bitterly cold temperatures.
The fire's cause is under investigation.