The Associated Press reports that the boy died Tuesday at a hospital.
The boy’s name was not immediately released. St. Louis police spokeswoman Leah Freeman says the boy was a ward of the state, and authorities still are working to notify his relatives.
Police say emergency responders were summoned to a home about 3 p.m. Tuesday for a report of a toddler in cardiac arrest. The boy was pronounced dead at a hospital about seven hours later.
It is standard protocol to have child abuse detectives investigate sudden deaths involving children.