Investigators have not determined if the shooter meant to shoot and kill the child or someone else at the home, but police say someone at the location was the intended target.
Armstrong seemed frustrated during his Wednesday address. He blamed the justice system for allowing repeat criminals to walk the streets.
“For whatever reason, he's out on our streets again, and this time has committed the ultimate act of murder. That murder of a 7-year-old child," Armstrong said.
District Attorney Amy Weirich says Tennessee sentencing laws determine prison time and probation.
"Most of these offenders are going to get out of custody," Weirich said. "It's a very small percentage of the criminal defendants we prosecute and put behind bars that actually spend the rest of their life behind bars."
Williams' visitation is Friday evening. Her funeral is set for Saturday at 11 a.m. Both will be held at Soul Winners Church in Hickory Hill
Williams is one of multiple children shot in the last week in Memphis.
The three men charged in the 7-year-old's murder are due in court Thursday morning.
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