Griffin could receive the death penalty or life in prison without parole.
The trial has been moved to Salisbury, 50 miles from the Union County town of Monroe, due to publicity.
Medlin, a cocktail waitress at a Charlotte club, was on her way home to her husband when she pulled to the side of the road at 3 a.m. on March 29. Her family and friends insist she would only have stopped for a police officer at that hour.
Prosecutors said that Griffin stopped her car, which he was using against regulations while off-duty and in uniform, and that the ensuing conversation led to his hitting her with his flashlight, choking her, stomping on her upper back, breaking her neck.
Medlin's body was left at the end of a cul-de-sac in an isolated industrial area, covered with debris.
Griffin's partisans say they cannot believe he would commit murder, and that they are not sure the state proved its case.
Prosecutors used the testimony of Griffin's former fellow officers and of people who crossed his path that night to piece together their scenario.
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