Currently, police are questioning Mya Lee's mother and her boyfriend. Her death was ruled a homicide.
Sunday night, the baby's uncle told ABC7 Chicago that Mya was in the care of her mother's boyfriend because she had to go to work.
"For all I know, something went wrong from the time she left for work that morning, and he took her to the hospital," uncle Joseph Jennings said.
No one was in custody, and no charges had been filed Sunday night.
Funeral arrangements had not been made public.