See, the crime never happened if those officially responsible for making that declaration say it didn't. An ME for example. They get to have "judgment calls" - the injuries were post-mortem caused by the body's contact with rocks/other objects driven by water currents. Easy.
Similar to how law enforcement in Chappaquiddick declared no crime had taken place when Teddy Kennedy drove his car off a narrow bridge while drunk, killing his passenger, and then didn't report it to authorities until the next day.