BOSTON (AP) — A dozen Tufts University lacrosse players were diagnosed with a rare muscle injury after participating in a voluntary, supervised 45-minute workout organized by the team, a university spokesman said Friday.

Five of the players diagnosed with rhabdomyolysis were still in the hospital Friday, according to Patrick Collins, the school’s director of media relations.

Monday’s training was led by a university graduate who had completed Navy SEAL training.

According to the Cleveland Clinic, rhabdomyolysis is a rare muscle injury in which a person’s muscles break down. It is a life-threatening condition that can occur after an injury or excessive exercise without rest.

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

“Our thoughts are with the players and their families and we hope they make a speedy recovery under the care of local medical experts,” Collins said in a written statement.

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

Given the severity of the injuries, the school is engaging an outside, independent investigator to conduct a thorough investigation as soon as possible with the goal of taking all necessary steps to protect the school’s student-athletes, he said.

The university is still investigating the circumstances that led to the injuries, he said, and cannot release further details until the investigation is complete.