University of Iowa leaders are ordering an investigation into what caused 13 football players to be hospitalized with an unusual muscle disorder following offseason workouts.

Board of Regents President David Miles and school President Sally Mason call the case "a cause for grave concern."

They agreed the university will have 90 days to complete an investigation analyzing events and the results will be presented to the Board of Regents. The review will involve independent experts.

The university acknowledged Wednesday the players are affected by rhabdomyolysis, a condition that involves the release of muscle fiber into the bloodstream and can cause kidney damage. It can be caused by physical exertion, and players were participating in workouts that started last week.