The child and his mother were found in a first floor room at the Diplomat Motel on the North Side around 10:30 Thursday morning.
Motel guest Ernest Randall was in the parking lot taking some items out of his vehicle when he says he saw a man run from the room next to his and into the street where he flagged down a passing squad on patrol.
"The man came running out of the room and said, 'She killed my baby,'" said Randall.
An employee at the Lincoln Avenue motel then called 911.
Officers found the baby dead in the bathroom of the room where the child, his mother, and a man are thought to have stayed since Tuesday.
Police officials say unconfirmed reports indicate that the woman stabbed the baby- then herself.
The incident is a shock to some residents, who say their Lincoln Square neighborhood has improved and doesn't have a lot of crime.