WARNING!: Description below may be disturbing to some readers.

The 1993 murder of 12-year old Polly Klaas is a well-known case that resulted in changes to the American court system.  More info below:

Polly Hannah Klaas (January 3, 1981 – October 1, 1993) was an American murder victim whose case garnered national media attention.  On October 1, 1993, she and two friends were having a slumber party. Around 10:30 pm, an intoxicated man named Richard Allen Davis unexpectedly entered her bedroom, carrying a knife from the kitchen. He told the girls that he was there to do no harm and was only there for money. Davis tied up both of her friends, pulled pillowcases over their heads, and told them to count to 1,000. He then kidnapped Klaas. Over the next two months, about 4,000 people helped search for Klaas. Actress Winona Ryder, who had been raised in Petaluma, offered a $200,000 reward for Klaas's safe return during the search. 

About two months later, Davis was arrested (he had prior convictions) and led the police to Klaas' body, which was buried in a shallow grave at the base of this sawdust burner in Cloverdale, California.  After a long and tumultuous trial, Davis was convicted on June 18, 1996, of first-degree murder with four special circumstances (robbery, burglary, kidnapping, and attempted lewd act on a child) in Klaas's death. A San Jose Superior Court jury returned a verdict of death. At his formal sentencing, Davis provoked national outrage by taunting his victim's family, extending both middle fingers at a courtroom camera and later saying that Klaas's last words just before he killed her implied that her father molested her. Judge Thomas Hastings then formally sentenced Davis to death, telling Davis that his conduct in the courtroom made the decision to pass the death sentence significantly easier. Davis has been on death row since 1996.

In the wake of the murder, Klaas' father, Marc Klaas, became a child advocate and established the Polly Klaas Foundation (formerly the KlaasKids Foundation).  The all-points bulletin was broadcast on the CHP (California Highway Patrol) channel, which only CHP radios could receive. CHP practice changed after the case. The radio system was upgraded and such bulletins are now broadcast on all police channels through a centralized 911 dispatch system.  In the wake of the murder, politicians in California and other U.S. states supported three strikes laws and California's Three Strikes act was signed into law on March 8, 1994.

From: https://en.wikipedia.org/wiki/Murder_of_Polly_Klaas