Three of the McMinnville High School players also were diagnosed with possible cases of a rare soft-tissue condition called "compartment syndrome," which caused painful soreness and extreme swelling in their triceps. They underwent surgery to relieve the pressure.
The 19 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 in McMinnville. High CK levels can lead to kidney failure if not properly treated.
"To have an epidemic like this is very weird," Winkler said.
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.
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.
Practices for all fall sports start Monday.
There were "two things going on here," CBS News Medical Correspondent Dr. Jennifer Ashton explained on "The Early Show" Monday. "One is a muscle breakdown process known as rabdomiolisis. That is not rare. We see that a lot with strenuous exercise. Athletes can see this.
"The rare thing here is compartment syndrome, which is going on with the upper extremity.
" … There are a lot of causes for this muscle breakdown. Certainly, dehydration, exercising in heat is one of them. Certain medications, even some dietary supplements have been associated with this. And certain infections and illness. There are a lot of causes. We do see this. It's not that commonly seen in healthy, high school athletes."
"It's a mystery," Ashton continued. "But this compartment syndrome, which affected three of these football players, where swelling occurs in the limbs, we usually see this after crushing trauma in the lower extremity. There are only ten case reports of upper extremity, of tricep compartment syndrome in the medical literature. It is exceedingly rare. It is usually treated with surgery. We don't know why these boys were affected with that. That's the real mystery."
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 that 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.
"We don't know," Ashton pointed out, "if they had a higher index of suspicion at this hospital because, once one or two boys went to the emergency room they started looking for it, testing for it. Other boys complained of symptoms. That certainly can be at play here. They're going to be following it. For most healthy people, this is not a real risk, so don't worry yet."
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."
A home phone listing for Kearin could not be found. But one of his former Cal State Northridge colleagues told The Oregonian that Kearin is "very conscientious about the high school development and the kids."
"His personality is not a big, hard-nosed, lineman's mentality, or a weight-room-mentality guy," Los Angeles Valley College coach Jim Fenwick said.
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."