The Tulare County District Attorney's Office says on August 12, 2008, Diaz was shot multiple times through his bedroom window at an apartment on E. Birch.
They say it was part of a gang crime.
Tulare police were able to get evidence, but no arrest was made.
In 2019, a Cold Case Homicide Unit was formed and a lead was found through DNA testing.
It was determined that 34-year-old Luis Villarreal was responsible for murdering Diaz.
When charges were filed on July 8, Villarreal was already in custody for recent misdemeanor convictions. He pleaded not guilty to the charge on Monday.
Villarreal is being held without bail and is scheduled to appear in court on July 18. He faces life in prison if convicted.