A witness said the boy, later identified as Marterrance Albury, was left in the street shortly before 10 p.m. Sunday. He died at a hospital early Monday.
The brother, 17, appeared 20 minutes later and told officers he was looking for the boy.
The family lives near where Marterrance was found, police said. The mother, Johntea Williams, 36, apparently left Marterrance with his brother while she went out with a friend.
Investigators searched Monday for security camera footage that might show what happened. They said they were unsure who was at the wheel of the Ford Expedition when Marterrance fell out and whether he might have been pushed out the door.
Williams became distraught and threatened suicide when she returned home at about 4 a.m. to learn that her son was dead, Laura McElroy, a police spokeswoman, said.