There's some more details on Amber's Charley Project page:
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Details of Disappearance
Amber was last seen in Maury County, Tennessee on April 11, 2004. She left her mother's home in the 2300 block of Tracy Lane with James Gordon, one of her friends, in a gold Mazda with the license plates number PFB-200. She was supposed to return in two days, but never did and has never been heard from again.
When questioned about Amber's disappearance, Gordon said he'd left her with another friend, Ronald "California" Inzurriaga. They were supposed to go to get a meal at the Waffle House restaurant on James Campbell Boulevard.
Authorities have since located Inzurriaga in a federal prison in Alabama. He said he dropped Amber off at a garage in Columbia, Tennessee after she purchased hair dye, and he doesn't know what happened to her after that.
Amber was born in Spring Hill, Tennessee and lived in several states, then returned to Tennessee to live in a foster home. Her older half-sister, Brenda James, stated Amber had gotten involved with the wrong crowd and was drinking and occasionally using drugs.
James became her legal guardian just a few days before she went missing.
Amber was originally considered to be a runaway, but the passage of time since her disappearance has led investigators to consider whether other factors caused her to disappear. She has a driver's license, but there have been no infractions listed on her driving record. Her Social Security number has gone unused since 2004, and she hasn't used her cellular phone either.
James believes Amber is deceased. If she's alive, she may be in Georgia or Texas, or she may still be in the Maury County area. Her case remains unsolved.
Amber Elizabeth Cates – The Charley Project