Steve Gehrke, spokesman for the Utah Department of Corrections, also confirmed Williamson's death.
Williamson, 30, pleaded guilty in 3rd District Court to first-degree felony murder in October 2010. In court testimony, Williamson said he agreed to drive around a group of teens affiliated with the Crips gang while they engaged in crime spree because he wanted to make some money.
Williamson admitted he drove Brandstatt to the golf course the night of Feb. 6, 2009, but claimed he did not know there was a plan to kill the teen who was trying to fit in with the Norteno gang but had not been initiated into the group for wearing a red T-shirt, the Norteno's color.
Antonie "Hunter" Farani, then just 14, shot Brandstatt.
Farani, now 18, was sentenced last year to a minimum of 35 years in prison, and possibly up to life, for murder and kidnapping.
Two others who were along that night and charged with participating in the slaying are also serving prison time.
Shardise "Kaiso" Malaga, 23, pleaded guilty to second-degree felony manslaughter and two first-degree felony aggravated kidnapping counts and is serving 15 years to life in prison.
Spencer Isaiah Cater, 22, was convicted of aggravated kidnapping and robbery charges, but acquitted of murder. Cater is serving 20 years to life in prison.