The Crab Island Murders

Alfonso was raised in a place much like this. He had a good mind for solving puzzles. He had just finished Police Academy, specialist in maritime law enforcement.
There were no open cases to be assigned. He was patrol, but had the option of investigating any cold case if he could find evidence that could lead to conviction.
He knew the area intimately in his two weeks on the job. He would get bored if he didn't find something to concentrate his talents on.
Then he found a file about four unsolved murders. On Crab Island.
It would be something to do.
Then things started getting weird. What had he stumbled onto? There was nothing on Crab Island but mangroves!