The SBI conducted a thorough investigation into Planten's suicide.
District Attorney Colon Willoughby says Planten appeared to be more interactive with guards in the days leading up to his death.
Planten hung himself inside Raleigh Central Prison in January.
Willoughby says there was nothing corrections officers could have done to stop him.
Planten was arrested in October for the 2002 murder of Stephanie Bennett.
He was also a suspect in the killing of a Michigan woman in 1999.