When police officers arrived at the 600 block of N. Wedgewood Avenue around midday, they found the 1-year-old in front of the structure. Investigators said it appeared the child may have been thrown from the apartment building.
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As more officers arrived on scene, a woman jumped off a second-floor balcony. When authorities went inside the building, they found another child - described as an infant girl - unresponsive in an apartment unit.
All three were transported to an area hospital in unknown condition. Police say the woman is the mother of the two children.
Later Tuesday, police said the girl found inside the apartment had skull fractures and other injuries and had died.
The 1-year-old was described as being in stable condition.
A manager of the apartment building identified the mother as Tierra Ortega. She was in the hospital and was expected to be arrested later.