After the announcement Tuesday that the connections of Kentucky Derby winner Sovereignty have opted to skip the Preakness Stakes on May 17, spoiling any slim chance of a Triple Crown sweep this year, hard questions are being asked about U.S. racing’s centerpiece series.

Thirteen horses have won the Triple Crown in American Thoroughbred racing history. Here's a look at the winners and the history of the Triple Crown:

Many think the traditional five-week Triple Crown rotation of Kentucky Derby, Preakness and Belmont needs a shake up as modern horse trainers do not like to race horses three times in five weeks.