On October 1, 1993 she was taken from her home at knifepoint in Petaluma.
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She was having a sleepover with her friends at the time.
Polly's body was found two months later.
Now, her murder has had a profound effect for missing children and crime prevention shaping groundbreaking measures in California and across the country.
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But, how did Polly's case transform the criminal justice system today?
Mark Klaas, Polly's father, recalled the moment he knew his daughter was gone talking with 20/20's JuJu Chang this week.
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"They didn't have to tell us anything Christ. They walked in and the tears were in their eyes," Mark said.
Decades later, he described the haunting memory.
"I remember that Eve (Klaas, Polly's mother) started crying, the cops were crying. I wasn't crying, but I said we need to tell our family, so I made a phone call and said, 'you know, it's over she's dead'."
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Richard Allen Davis was identified as Polly's killer. He led investigators to where he left the young girl's body.
Davis was sentenced to death, but the death penalty is on hold in California.
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Marc Klass moved to action starting the Polly Klass Foundation, and helping to transform criminal justice.
"The Polly Klaas murder case was a huge shift in the criminal justice system across the United States. It was the beginning of the victims rights movement," said Steven Clark, a legal analyst and says the three strikes law is among the most notable of changes.
"Richard Allen Davis was a career criminal and he was out of jail and what three strikes law did was, we're going to be putting people away forever who commit a series of violent felonies," Clark said.
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The case is also credited with helping lead to the establishing of the Amber Alert.
"The Polly Klaas case really spoke to a lot of change in law enforcement ...and how they conducted these investigations including Amber alerts so people could quickly respond to a kidnapped child," he added.
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