University of Iowa leaders promised Thursday to investigate what caused 13 football players to be hospitalized with an unusual muscle disorder following offseason workouts.

Board of Regents President David Miles and Iowa President Sally Mason called the incident "a cause for grave concern." The university will have 90 days to complete an investigation analyzing events leading up to the hospitalizations, and the results will be presented to the board.

The review will involve independent medical experts.

"It is essential that we take the necessary steps to understand the factors that led to this to ensure that it never happens again," Miles said.

The university acknowledged Wednesday that the players have rhabdomyolysis, a condition involving the release of muscle fiber into the bloodstream. The disorder, which can cause kidney damage and even failure in severe cases, can be caused by physical exertion.

The players were participating in grueling workouts that started last week.

"It is an essential responsibility of the University to determine what is likely to have caused this rare condition among so many young men at one time, and to share those findings," Mason said.

School officials say the players were in stable condition and responding well to treatment, which includes bed rest and the administration of hydrating fluids.

Director of football operations Paul Federici said the players participated in workouts that started last Thursday after they returned from winter break. Some complained to medical staff after a workout Monday and symptoms included soreness throughout the body and tea-colored urine.

Iowa coach Kirk Ferentz and athletic director Gary Barta did not attend Wednesday's press conference, and spokesman Phil Haddy told the Associated Press on Thursday morning that there aren't any current plans for Ferentz to meet with the media before recruit signing day on Feb. 2.