School officials, parents and players remained puzzled Sunday over what caused 19 members of a high school football team in northwest Oregon to suffer muscle damage — three of whom required surgery — following a fall camp.

The 19 McMinnville High School players all had elevated levels of the enzyme creatine kinase, or CK, which is released by muscles when they’re injured, said Dr. Craig Winkler of Willamette Valley Medical Center. High CK levels can lead to kidney failure if not properly treated.

‘‘To have an epidemic like this is very weird,’’ Winkler said. ‘‘To have this number of athletes in one single community in one environment is very usual.’’

Three of the players also were diagnosed with a rare soft-tissue condition called ‘‘compartment syndrome,’’ which caused soreness and swelling in their triceps. They underwent surgery to relieve the pressure, Winkler said.

Five of the athletes were treated in the emergency room and sent home. The other 11 were admitted to the hospital and given intravenous fluids to maintain adequate hydration and prevent kidney failure, he said.

Ten boys remained hospitalized Sunday, but they were in good condition and were expected to be released Monday, said Rosemari Davis, Willamette Valley Medical Center’s chief executive officer.

Officials said the cause was still a mystery, but high CK levels can result from vigorous exercise or the use of certain medications or food supplements.

Before their symptoms started this past week, the players were at an immersion camp organized by first-year coach Jeff Kearin. Winkler said the players worked out last Sunday at the high school’s wrestling room, where temperatures reached 115 degrees.

He said the high temperature and dehydration may have played a role. He also said officials will look at water sources and what the kids had to drink, including power mixes.

Winkler said blood test results expected Tuesday could show whether the athletes ingested creatine, which is found in legal high-powered protein supplements. He added officials are not testing for steroids because it would be unlikely for that many students to have access, and ‘‘creatine makes way more sense.’’

Two players said Sunday they were unsure what caused their injuries but supplements were not a factor.

Fullback and linebacker Jacob Montgomery, one of the 10 still hospitalized, said he first experienced a tightness in his triceps and forearms Tuesday.

‘‘They swelled to the verge of popping,’’ the 17-year-old senior said in a telephone interview. ‘‘I thought it was just swelling from an intense workout.’’

Montgomery said he went to get checked out Wednesday after learning another player was taken to the hospital.

He and fellow senior Josh Nice said neither they nor any of the other players have taken any supplements or performance enhancers.

‘‘They don’t know what’s behind this whole thing,’’ said Nice, a wide receiver hospitalized since Friday. He added he hopes to return to practice as soon as possible.

Winkler said the hospital and school began screening players for CK after the first few were brought to the hospital early last week.

The normal range for CK is 35 to 232 units per liter, but some students showed levels as high as 42,000, putting them at risk of kidney injury, Winkler said. Those with levels in the 3,000 range were treated in the hospital’s emergency room and released, while those with levels above 10,000 were admitted.

Superintendent Maryalice Russell told The Oregonian newspaper she doesn’t believe Kearin’s workout was excessive. She also said she has no evidence steroids or supplements were involved.

‘‘I don’t have any information at this time that would indicate that’s the case,’’ she said. ‘‘I’m continuing to look at additional information as it may come my way.’’

Tom Welter, Oregon School Activities Association executive director, said the organization’s medical committee will investigate and make recommendations to the executive board after its next meeting in September. The OSAA oversees school sports in the state.

‘‘It’s a really bizarre situation,’’ said Nice’s mother, Margaret Nice, whose son Daniel also remains hospitalized. ‘‘But we’re all trying to hang in here and hope and pray that they can come up with the answer to what caused this.’’

Practices for all fall sports start Monday.

‘‘We just want our boys to get better, and they’re all anxious to get out on the field,’’ Margaret Nice said.