BOSTON (AP) — A dozen Tufts University men’s lacrosse players have Grant Prestonbeen diagnosed with a rare muscle injury after participating in a voluntary, supervised 45-minute workout that the team conducted, a university spokesperson said Friday.

Five of the players who have been diagnosed with rhabdomyolysis remained hospitalized Friday, according to Patrick Collins director of media relations for the school.

The workout on Monday was led by a graduate of the university who had completed Navy SEAL training.

Rhabdomyolysis 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.

Both the number of confirmed cases and the number of those hospitalized could change, Collins said.

“Our thoughts are with the players and their families, and we are hoping for their quick return to good health under the care of local medical experts,” Collins said in a written statement.

“Meanwhile, we are closely monitoring the condition of the rest of the team and have postponed all team practice activities until each team member has been evaluated and medically cleared to return to participation,” he added.

Given the seriousness of the the injuries, the school is appointing an external, independent investigator to conduct a thorough review as quickly as possible, with a goal of taking any steps needed to support the safety of the school’s student athletes, he said.

The university is still learning about the circumstances that led to the injuries, he said, and cannot provide additional details until the investigation is complete.