The incident happened in the area of Nelson and Garden streets in Hartford.
Police say 3-year-old Rondell Jones was riding in a car with his mother, two young siblings and another man when a Honda Accord pulled alongside and began firing.
Authorities believe the man in Jones's car was the intended target. No one else in the car was hurt.
Both the shooter and the intended target fled after the shooting and had not been located Saturday night, police said.
The car, which police said had been stolen from Windsor Locks, was found unoccupied an hour later.
The stolen car used in the shooting was found abandoned.
Police say the car the 3-year-old was in took off and brought the child to a hospital.
The child was later pronounced dead.
"For anyone with a heart, a soul, anyone who's ever loved a child, this is a crime that is unfathomable and sickening," Mayor Luke Bronin said at a news conference. "I am heartbroken, and I am angry. When you watch the video, it is difficult to believe that the shooter did not know who he was shooting at, and it is very difficult to believe he could not see children in that car."
About two hours later, another shooting in Hartford claimed the life of a 16-year-old.
"This is a time where I believe there's a call to action," Hartford City Council Majority Leader T.J. Clarke said, "for anyone who has witnessed this heinous act to step up and please break the 'street code' by saying what they witnessed."
Anyone with information is asked to call police at 860-722-TIPS (8477).
The Associated Press contributed to this report.
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