Excerpts from abc7chicago.com:

Three people were injured, two critically and the third was in serious condition from an apartment fire Tuesday evening in the 4600-block of North Sheridan, a Chicago Housing Authority senior building called the Ella Flagg Young Apartments.

The fire was contained to a fourth floor unit, which was gutted, but sent smoke up several floors.

The immediate concern for firefighters was the mobility of the elderly residents, and if they needed to be evacuated. 

The fire was extinguished quickly with the help of hallway sprinklers and most of the residents sheltered in their units, but some chose to evacuate.

The cause of the fire remains under investigation.