4:18 PM PT -- A rep from the Union Parish Sheriff's Office tells us, "There is an active investigation at the prison's request. We are currently awaiting the results of an autopsy. The initial investigation did not suspect foul play. We are also conducting an investigation into the fight which occurred earlier and Porter's involvement in that incident."
The statement continues, "We respect the family's right to request another autopsy to be done. This is not an unusual request. We are continuing our investigation and awaiting autopsy results."
Tyler Perry is not convinced his nephew hanged himself in prison, and he's hired a famous medical examiner to determine if he committed suicide or if he was killed in his cell.
TMZ broke the story ... Tyler's nephew, 26-year-old Gavin Porter, was serving 20 years for shooting and killing his dad in front of his mom -- who's Tyler's sister. He was found Tuesday night hanging in his cell from a bedsheet.
One of the reasons Tyler is suspicious ... Gavin got into a fight with another inmate last weekend and was placed in solitary.
Tyler tells TMZ, "Call me naive, but it was my hope that after serving his time, and really reflecting and showing much remorse and asking God for forgiveness, that he would have been able to come to work for me, joining all the other former inmates who work for me and turn his life around just as they have. But that day will never come."
Tyler says he's hired Dr. Michael Baden, a famed pathologist who has been involved in numerous cases, including JFK, O.J. and Jeffrey Epstein. Dr. Baden will perform a second autopsy.
As for why, Tyler says, "I want to be clear that we are not a family of conspiracy theorists and want to believe that there was no foul play just as the sheriff said publicly."