SAN ANTONIO (July 20, 2009)--A 13-year-old boy died after taking a car that belonged to his uncle and crashing in a rollover accident.

Authorities say it is unclear why the teenager took the keys and drove away Sunday night in Lytle, which is just outside of San Antonio.

Bexar County Sheriff's Sgt. E.M. Conger said it appeared the underage driver was being followed by an 18-wheeler that his uncle was driving.

Conger told the San Antonio Express-News that the uncle left the scene at some point and was not there when investigators arrived.

Authorities say the boy was traveling at an unknown speed when he lost control of the car, which crashed into a gate and rolled.