Authorities in South Carolina say the mother of the child is now in custody, WSOC-TV reports.
Chesterfield County Sheriff Jay Brooks said on Tuesday afternoon he doesn't believe there was a kidnapping, as reported to deputies. The suspect is being questioned in the death of the child.
The Chesterfield woman originally told police she was walking to check her mailbox around 2 p.m. when a tan and gold colored SUV stopped in the middle of the road.
At that point, she said, a man wearing a black coat, toboggan and one black glove got out of the vehicle and punched her multiple times in the head.
She said the man then grabbed her baby and took off in the SUV.
An Amber Alert was issued for the child a short time later.
Sheriff Brooks said the child was found dead in a diaper box in a field about 1,000 yards behind the family's home.