HONOLULU -- A University of Hawaii freshman swimmer who drowned during practice suffered from a heart condition, an autopsy revealed Wednesday.

Mike Sheldt, 18, had hypertrophic cardiomyopathy, a disorder of the heart muscle that generally includes enlargement of the heart and a thickening of the walls of the left ventricle, the city medical examiner's office said.

The disorder was a "contributing factor'' in his drowning Tuesday, the office said.

Coach Mike Anderson and team physician Dr. Andrew Nichols said they had not been aware Sheldt had any medical problems. They were also unaware of any drugs or supplements he might have been taking, noting that there are rigorous NCAA guidelines.

"All the kids are required to undergo a thorough screening with our medical staff before the season starts,'' Anderson said. "Both a long, involved questionnaire and a regular physical with a doctor. And there was nothing that indicated, at that time, any preexisting conditions.''

Sheldt is the first Hawaii athlete to die during practice or an event in at least 30 years.

Teammates pulled Sheldt, of Charlotte, N.C., from the bottom of the pool at the school's Duke Kahanamoku Aquatics Complex and attempted to resuscitate him. He was taken to Straub Clinic & Hospital, where he was pronounced dead.

"The training partner noticed he'd gone down,'' athletics director Herman Frazier said. "I had heard perhaps 15 seconds or so and we reacted and responded as quickly as possible.''

Nichols said a portable defibrillator was also used in the attempt to revive Sheldt. Nichols would not comment on Sheldt's vital signs.

"Certainly we had no indication that he had any type of serious ongoing problem that predisposed him to these tragic conditions,'' he said.

There were more than 20 people in or around the pool during the incident. Frazier said safety precautions were followed "by the book.''

School officials have counseled Sheldt's grieving teammates, who practiced Wednesday.

"This is what Mike would've wanted -- is his teammates to go on without him,'' Anderson said. "It's with a broken heart and a lot of sadness. Our kids and our coaches, we're showing some cracks, but we're not broken and we're going to go on.''

Sheldt swam in the 200 and 400 individual medleys and was one of Hawaii's rising stars.

"I knew Mike well and he was a promising young athlete and scholar,'' university president Evan Dobelle said. "His loss is personally devastating to me, and I am struggling to make sense of a situation that has no explanation.''

In high school, Sheldt was a state champion swimmer in North Carolina and a member of the National Honor Society.

"We are all shocked. We are all numb from the loss of Mike Sheldt,'' Anderson said. "He loved Hawaii and loved being with our team. Loved to surf. Loved to swim. And loved his teammates.''

Sheldt's parents arrived in Honolulu on Wednesday evening.