Federal investigators have closed their investigation into the death of Derek Williams. The 22-year-old died while in police custody on July 6, 2011.
A squad car video showed Williams gasping to breathe in the back seat and officers failing to answer his calls for help.
Officers had chased him for several blocks, wanting to question him about a robbery.
According to the U.S. Attorney's Office in Milwaukee, prosecutors did not believe they could prove, beyond a reasonable doubt, that any officer willfully deprived Williams of a constitutional right. So they will not file civil rights charges against police.
The federal office reports telling the Williams' family of the decision earlier Tuesday.
Investigators say they interviewed more than 50 witnesses and medical experts and analyzed all data collected, but could find no conclusive evidence that officers used unreasonable force in arresting Williams or failed to respond to Williams' medical need when they noticed he was in distress.