In the days after a 45-minute workout led by a Tufts graduate turned Navy SEAL, members of the men’s lacrosse team became ill and nine were hospitalized with a serious muscle condition. On Wednesday, nine days after the voluntary workout, the last three players were released, and the university’s independent investigation began in earnest.

The players were treated after developing rhabdomyolysis, also known as rhabdo, a serious and rare muscle condition. It is unclear what the workout entailed. The person who led it is an active duty Navy SEAL, a Naval Special Warfare spokeswoman said. The SEAL was not there as part of a Navy-sanctioned event, and it was unclear if the sailor would face any disciplinary action, the spokeswoman said.

The workout on Sept. 16 was voluntary and supervised, according to Patrick Collins, a Tufts spokesman, and about 50 players participated. Tufts declined to say who supervised the workout.

Men’s lacrosse practices are postponed until the university’s medical staff decides they can resume, Mr. Collins said in an email on Wednesday. But some individual athletes have been cleared to continue training.

On Wednesday, Tufts announced that it had appointed two independent investigators to look into what had happened and how to prevent it from recurring. The university did not provide a timeline for the investigation but said that it would give the investigators “all the time and information” they need.

The Division III team, coached by Casey D’Annolfo, has won several N.C.A.A. national championships, including in May, when it defeated Rochester Institute of Technology.