McMINNVILLE, Ore. (AP) — Doctors were trying to determine why 24 McMinnville High School players developed a condition that caused intense pain and dangerous swelling in their arms and hospitalized two boys just as a football training camp was getting under way.

The athletes were taking part in the immersion camp organized last week by new coach Jeff Kearin to get ready for the season. Some were present for an intense drill session on Aug. 15, the night before camp opened.

Authorities say 11 players were treated in the emergency room last week, 13 were admitted and three required surgery to relieve pressure caused by the swelling.

Physicians still don’t know why so many players came down with the condition, called compartment syndrome, which is marked by high enzyme levels in the blood and can be caused by certain drugs or by strenuous exercise, dehydration or a combination of factors.

Kearin said at a news conference Monday — the same day the team opened fall football practice — that he and his staff were not aware that any players “knowingly took anything that would cause them harm.”

Devin Draper, a sophomore left guard, did not come down with any symptoms, but said Monday that teammates were as perplexed as anyone by the rash of players who succumbed.

“I just want to know what happened,” Draper said. He was one of just about 19 players who were able to report to fall practice when it opened Monday at the school’s campus, about 40 miles south of Portland. The team has about 45 players in all.

Dr. Craig Winkler, who is one of those treating the athletes, said the results of blood tests are expected Tuesday or Wednesday and could determine whether any player took a supplement — like creatine — that could explain what happened. Some supplements can increase enzyme levels in the blood when combined with exercise and dehydration, he said.

“A few of the kids did admit they were drinking protein shakes, but we don’t know what was in them,” Winkler said Monday.

McMinnville School District Superintendent Maryalice Russell said one parent brought in the label of a protein shake a player had been drinking. Many of the athletes who were not hospitalized submitted to blood tests.

Winkler said it seems unlikely the players organized the use of a substance. “Usually, there’s going to be one kid who’s going to squeal, but we have heard nothing,” the doctor said.

Many players on the team were taking part in the immersion camp, during which players were going to stay overnight while training during the day at the school of about 1,700 students.

But several arrived early on the night of Aug. 15 and took part in an intense drill in the high school’s wrestling room, where Winkler said the temperature reportedly reached 115. He said the players had access to water at intervals, although they didn’t have water bottles nearby. “Some of the kids did drink, some didn’t,” he said.

The room was not air conditioned. Members of the coaching staff who were on hand did not say the room was uncomfortably hot, Russell said.

McMinnville Police Capt. Dennis Marks said his force hasn’t begun an investigation.

“We haven’t gotten any information from the school or medical officials to give us reason to open one at this point,” he said.

Kearin, a veteran of nearly three decades at the high school and college level in California, spoke briefly at Monday’s news conference, reading from a prepared statement.

He described routine exercises in which players work with a partner. He did not answer questions.

McMinnville High’s campus was closed off to visitors, and two police cars were parked in the parking lot or Wartman Stadium, where practice was held.

Draper said many players were concerned that the team would be tainted by the suggestion that steroids or other performance-enhancing supplements were involved.

However, he expected such adversity would bring the team together.

“The worst has already happened,” Draper said. “It will only get better from here, I think.”

The athletes who were hospitalized will have to get signed permission from their doctors to return to practice. There is a chance the Grizzlies will have to forfeit their season opener.

Kearin has been head coach at Cal State Northridge, which cut football in 2001. Most recently, he coached Loyola High School in Los Angeles to the Southern Section Division I championship before leaving the post in 2009.

Adam Guerra, who worked as an assistant under Kearin at Loyola and took over as Loyola’s interim coach last year, said Kearin “always put his players’ well-being as a top priority.”

“He was never a rule-breaker. I would describe him as a player’s coach. He was not one of those old-school, smash-mouth kind of guys,” Guerra said in a telephone interview from Los Angeles, where he is taking a year off from football.

He said Kearin never encouraged players to use supplements to build strength, believing that good conditioning and good nutrition were enough.

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Associated Press writer Jeff Barnard in Grants Pass contributed to this report.