15-year-old Candice Parchment went missing in April of 2010.  Police found her remains beneath a mattress in a wooded and secluded area in November of 2010.  They finally got a break in the case when they read Candice's own diary where she described a previous attack by two other teenagers, Marshae Hickman and Jermaine Robinson.   
https://www.drinkingthecoolaid.com/

Resources:
Who was Candice Parchment and how did she die? | The US Sun (the-sun.com) 

Candice Parchment Murder: Where are Marshae Hickman & Jermaine Robinson Now? Update (thecinemaholic.com) 

Man found guilty of murdering teenage girl – WSB-TV Channel 2 - Atlanta (wsbtv.com) 

Five most chilling Candice Parchment murder details revealed in documentary from dark diary entry to violent attack | The Sun 

Hickman v. Georgia :: 2016 :: Supreme Court of Georgia Decisions :: Georgia Case Law :: Georgia Law :: US Law :: Justia 

Watched: How I Caught My Killer Season 1 Ep 2 I found Candice’s Diary on Hulu 

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