Jackson's cousin, Tondra Mosley of Los Angeles, said the woman worked as a sales representative at a Macy's store and lived with her child's father, Craig Prom, in Oklahoma City.
Relatives became worried when they couldn't reach Jackson by cellphone on Friday after the baby, Donavon Prom, was found in a car seat in the heated apartment hallway. Jackson had no known connection to the apartment building.
Relatives said Jackson left her Oklahoma City home on Jan. 2. Craig Prom found a note saying she needed more space.
She went to her aunt's home in East St. Louis on Thursday, but no one was home. She and the baby visited a neighbor and she left about 10 p.m. Thursday, telling the neighbor she needed to find a place to stay for the night.
Jackson then called Mosley's mother saying she would stop by on Friday with the baby. That was the last relatives heard from her.
Linda Lawson, a family spokeswoman, issued a statement describing the family's "unimaginable heartache."
"Someone out there knows what soulless people did this: Please come forward to let us know before it's your loved one," the statement said.
Copyright 2013 The Associated Press.