Grinstead, an Irwin County High School teacher, left the Sweet Potato Festival and headed to a cookout with friends.
Lindsey told Channel 2 Action News he has been sharing new leads and information with a private investigator, who worked with Grinstead's family shortly after her disappearance.
Ricketson said the GBI took out a warrant Thursday charging Duke with one count of murder.
A news release from the GBI said foul play was immediately suspected and a massive search was conducted.
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The GBI said the case has "never gone cold". They found her cell phone and vehicle all still there but the 30-year-old teacher was gone. Authorities did not provide very many details about any prior relationship between Grinstead and Duke, except to say that Duke was a student at the same high school where Grinstead was a teacher.
The GBI declined to say how they believe Grinstead died or if they have found her remains. Though they have received many tips over the years, none led to credible information.
"This gentleman never came up in our radar", J.T. Ricketson, special agent for the Georgia Bureau of Investigation, said of the near cold case in a press conference Thursday. Her purse and keys were missing.
He said Duke was arrested Wednesday and is now in the Irwin County jail.