Matthew Sell, a doctor who treated newborn infants at Saint Alphonsus Regional Medical Center in Boise, died Saturday while swimming in a triathlon in Emmett.

Sell was about 50 yards from the finish of the 0.9-mile swim portion of the race, said Ken Runyan, director of the Emmett's Most Excellent Triathlon.

There were 24 kayaks in the water to monitor the swimmers and 12 lifeguards on hand, Runyan said.

"When he quit swimming, they were on him almost immediately and had a breathing device on him," he said. Sell was flown by air ambulance to Walter Knox Memorial Hospital in Emmett, where doctors reported he likely had died from cardiac arrest, Runyan said.

The Olympic-distance triathlon in which Sell competed included a 6.2-mile run and 24-mile bike race. It started with the swim event at 9 a.m. at Black Canyon Park. The races continued as planned, Runyan said.

Sell was a graduate of University of Wisconsin Medical School. According to his state profile, he started working at Saint Alphonsus in 1998. He previously worked at St. Luke's Regional Medical Center, starting in 1985, according to his profile.

Sell "saved the lives of countless babies during his many years working in the Saint Alphonsus neonatal intensive care unit," said Sally Jeffcoat, president and chief executive officer of Saint Alphonsus Health System. "His unwavering dedication to his patients and his kind and gentle demeanor will be sorely missed. The entire Saint Alphonsus Health System staff sends our deep condolences and prayers to the Sell family."

Sell was in his late 50s, according to the hospital.

This was the 11th year of the annual Emmett triathlon. There were about 500 competitors in the triathlon, Runyan said.