BOSTON (AP) — Three Tufts University men’s lacrosse players remained hospitalized with a rare muscle injury Monday after participating in a voluntary,Johnathan Walker supervised 45-minute workout earlier this month, according to a university spokesperson.

The players became ill in the days following a Sept. 16 workout that was led by a Tufts alum who is a recent graduate of the BUD/S Navy SEAL training program.

About 50 team members participated. All were evaluated by medical professionals, with a total of nine requiring hospitalization for rhabdomyolysis, according to Patrick Collins, the university’s director of media relations.

The three who were still hospitalized Monday were responding to treatment and were expected to be discharged soon, he said. Some team members have been medically cleared to resume training. All team practices have been postponed pending authorization from university medical personnel.

Rhabdomyolysis, also known as rhabdo, is a rare muscle injury where a person’s muscles break down, according to the Cleveland Clinic. It’s a life-threatening condition that can happen after an injury or excessive exercise without rest.

The school is appointing an independent investigator to conduct a probe into what happened before, during, and after the workout; assess the university’s response; and review its procedures and policies to determine what changes it should make to prevent this from happening again, Collins said.

“The team is a tight-knit group of young men who have shown remarkable resilience, understanding and care for each other throughout this episode,” Collins said in a written statement. “We will continue to monitor and work with them closely, and we hope for a rapid return to good health for all involved.”

In 2011, 13 Iowa football players were hospitalized for rhabdo after an offseason workout, and in 2016 the university paid $15,000 to settle a lawsuit brought by one of the players.