22-year-old Tahmir Craig was released last month.
He was charged in the Memorial Day murder of a pastor's son.
But FBI evidence later cleared him of all charges in the killing of 27-year-old Devon Williams.
Craig's attorney, Joseph Oxman, says he's now suing Delaware County, the City of Chester and four of the police detectives involved.
Oxman says leads that could have cleared Craig were not followed.
"They had evidence in hand that was readily available and that if they used basic police practices to investigate this, Tahmir Craig would never have been charged," Oxman said.
Delaware County district attorney Jack Whelan has said he met with Craig and his mother and apologized for the error.