Why Is Nolan Wells' Case Now Headed To A Grand Jury?

A district attorney doesn't send a drowning to a grand jury. So why is that exactly what's happening in the Nolan Wells case? 
Nolan was eighteen, a wide receiver at Southwest Mississippi Community College from Ocean Springs, Mississippi. On the Fourth of July he went out to Horn Island, an uninhabited barrier island roughly ten miles off the Mississippi coast, with a group of friends. No buildings, no fresh water, no service, just boats until dark, and then everyone heads back to the mainland. His friends made it home. He didn't. 
A National Park Service ranger found hi...