Multiple Tufts lacrosse players were diagnosed with a rare, sometimes life-threatening muscle injury after a 45-minute workout last week.

Three players are still hospitalized a week after the workout, a spokesperson for the school said on Monday.

Twelve members of the men’s lacrosse team were diagnosed with rhabdomyolysis, or rhabdo, after a workout on Sept. 16, Tufts spokesperson Patrick Collins said in a statement. The workout was led by a Tufts alumnus who had recently graduated from a Navy SEAL training program.

Shruti Gupta, director of onconephrology and a physician at Brigham and Women’s Hospital, said rhabdo is a syndrome where muscle cells break down and enter the bloodstream, which causes damage to multiple organs, particularly the kidneys.

Hospitalized cases most likely involve high levels of creatine kinase, an enzyme.

“The sense is probably that some of them might have really, really, really high levels of this creatine kinase, and that in turn signifies more intense muscle damage, and therefore more intense damage to the kidneys and other organs,” Gupta said.

Five of the Tufts lacrosse players were in the hospital as of Friday, and three remained hospitalized on Monday. The university said they are appointing an independent investigator “to determine exactly what happened before, during, and after the workout” and evaluate Tufts’s response.

“I think it’s unusual for particularly presumably young, healthy, physically fit males to, first of all, be hospitalized at all with this and then, let alone for this long,” Gupta told Boston.com. “One has to suspect that potentially, there is some serious organ damage that may have occurred.”

Gupta said overexertion, which is the presumed cause of the Tufts players’ rhabdo, is not the most common cause of the condition. She said it’s more common for rhabdo to appear after trauma or crush injuries, excessive heat exposure, certain medications, or with underlying genetic conditions.

The content of the workout has not been disclosed. Gupta said it’s “really, really rare” for multiple athletes in a group to get rhabdo at once. While she hasn’t worked on this particular case, she said short burst interval training and CrossFit-type workouts can sometimes lead to the condition.

Gupta said the players may have experienced symptoms like extreme muscle soreness, stiffness, or weakness, and brown-colored urine within 24 to 48 hours after the workout.

Death or disability from rhabdo is highly unlikely, Gupta said, particularly in young, healthy people.

“The prognosis is very good in the sense that people do tend to have a full recovery, even if they have kidney injury requiring dialysis. Most people do recover back to their previous baseline kidney function,” she said.