The University of Iowa already has begun an investigation into why 13 of the school's football players were hospitalized this week for a rare muscle disorder called rhabdomyolysis.

David Miles, president of the Iowa Board of Regents, said that he and university president Sally Mason agreed on a 90-day timetable for the investigation, which he said Mason initiated shortly after the players were admitted to University Hospitals on Monday.

"The primary aim of this analysis will be to identify, to the extent possible, the root causes of this incident in order to create and implement effective preventative measures to ensure this does not happen in the future," Mason said. "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."

Both Mason and Miles expressed "grave concern" for the athletes involved.

"Going forward, 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 added.

All 13 players remained in the hospital Thursday and continue to receive treatment, according to Tom Moore, the hospital's media relations coordinator.

He said he was uncertain when they might be discharged, referring to comments made by Dr. John Stokes at a Wednesday news conference that the players could be in the hospital for as much as a week.

The Sporting News reported Thursday that all 13 players were tested for illegal drugs and the results came back negative.

The publication, citing anonymous sources, also reported that head coach Kirk Ferentz returned from a recruiting trip Wednesday night and was confronted by a group of angry parents. Ferentz still has not made any public comment on the situation.

The players were hospitalized Monday after complaints of extreme muscle soreness and irregular urination. They have been treated with bed rest and IV fluids.

Stokes, who is not directly involved in the treatment of the players, said Wednesday he was unsure if any of the athletes had undergone dialysis to deal with the kidney malfunctions that can come with rhabdomyolysis.

The identities of the players have not been revealed because of federal privacy laws, but one is known to be freshman linebacker Jim Poggi, whose father also appeared at the Wednesday news conference. Other players who have been mentioned on Facebook pages as being hospitalized include linebacker Shane DiBona, cornerback Willie Lowe and fullback Jake Reisen.

The massive outbreak of rhabdomyolysis is highly unusual, according to health experts, but not unprecedented.

A high school football team in McMinnville, Ore., reportedly had as many as 24 players diagnosed with the syndrome last summer. Three of those players had to undergo emergency surgery for compartment syndrome, which occurs when the flow of blood and oxygen to the muscles is impaired by pressure. Doctors in the McMinnville case found no evidence that the players took steroids or other harmful supplements and attributed the problems to excessive workouts in 115-degree heat.

A British medical journal reported in 2005 that more than 100 high school students in Taiwan developed rhabdomyolysis after a physical education teacher required them to do 120 push-ups in 5 minutes.