Marvin Banks faced two counts
of capital murder charges in the deaths of Hattiesburg officers Benjamin Deen and Liquori Tate in May.
Banks was found unconscious in his cell, where emergency crews performed CPR, said investigator Nick Calico of the Forrest County Sheriff's Department.
He was rushed to a hospital, where he was pronounced dead Friday night.
Deen, 34, and Tate, 24
, were making a traffic stop
on May 9 when they were shot. Four suspects fled the crime scene after the shooting, allegedly stealing a police cruiser and using it as a getaway car, authorities said at the time.
In addition to the capital murder accusations, Banks, 29, faced grand theft auto and firearm charges.
His brother, Curtis Banks, was charged with two counts of accessory after the fact of capital murder. A third suspect, Cornelius Clark, was charged with obstruction of justice.
Another suspect, Joanie Calloway, 22, was charged with two counts of capital murder, authorities said.
The Mississippi Bureau of Investigations will oversee Banks' autopsy. Banks was in an isolation cell and there were no immediate signs of foul play, Calico said.