The fire in the 2800-block of West Byron Street broke out just before 7 a.m. in a fourth-floor unit, and was extinguished by about 7:35 a.m., CFD tweeted.
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Three people were taken to a local hospital, including the person living in the unit where the fire broke out.
That individual was in critical condition Tuesday morning.
Two other people were taken to a local hospital, but are expected to be OK.
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Firefighters said other residents of the building will be able to return to their homes.
The cause of the blaze is under investigation.
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Some smoke was visible while firefighters battled the fire.