The breaststroker Michael Alexandrov looked up video of Alexander Dale Oen on YouTube last week. The United States Olympic trials are in June, and Alexandrov was studying Dale Oen’s technique to see how to improve his own stroke.


Alexandrov’s training partners at the University of Southern California include Kosuke Kitajima, who is regarded as the best breaststroker in history, but Alexandrov considered Dale Oen, the reigning world champion in the 100-meter breaststroke, to be the man to beat at the London Games.


On Tuesday, Alexandrov was on his way to a morning workout when he received an e-mail from a friend with the news that Dale Oen died Monday from cardiac arrest while in Flagstaff, Ariz., for high-altitude training with other members of the Norwegian national team. Dale Oen was 26.


According to The Associated Press, a Norwegian team doctor performed CPR on Dale Oen after he was found on his bathroom floor. He was taken by ambulance to the Flagstaff Medical Center before being pronounced dead.


The Flagstaff Police Department is investigating Dale Oen’s death. According to The A.P., the police said there were no signs of trauma or foul play.


“I’m just really torn apart,” Alexandrov said in a telephone interview. “He was a great guy, and I considered him the favorite for the gold.”


The death of Dale Oen, considered one of Norway’s top Olympic medal prospects in London, dominated the headlines in his homeland. On Twitter, the Norwegian prime minister, Jens Stoltenberg, described Dale Oen as “a great sportsman for a small country.”


At last summer’s world championships in Shanghai, Dale Oen turned in the most emotionally charged performance of the meet. Competing in the 100 breaststroke final three days after 77 people, mostly children, died in the worst massacre in Norway’s history, he won in 58.71 seconds. It was the fastest time recorded by a swimmer not wearing the now-banned polyurethane suits and the fourth fastest in history.


After his time flashed on the scoreboard, Dale Oen pointed to the Norwegian flag on his cap, rose from the water and flexed his biceps in a show of strength to those back home in Norway.


“We need to stay united,” he said after the race. “Everyone back home now is of course paralyzed with what happened, but it was important for me to symbolize that even though I’m here in China, I’m able to feel the same emotions.”


Mark Gangloff, an American who finished eighth in the race, described Dale Oen’s victory as “one of the most amazing performances ever.”


“He really put his country on his shoulders for that race,” said Gangloff, who was nearly two seconds off Dale Oen’s pace. “It was a rallying point for that country to come together.”


Since last summer, Gangloff said, he has watched the video of the final several times — most recently last week.


“I’ve looked at it to see what I could have done better, but also to see what he was doing correctly,” Gangloff said by telephone from Auburn, Ala., where he trains. “The way he swam that race was amazing — he didn’t hold anything back. He just went for it. His swim set the standard for the rest of us.”


Alexandrov said Dale Oen’s technique, which stood out because of his high stroke rate, was frequently copied but never duplicated.


“He was like a feather in the water,” said Alexandrov, who represented Bulgaria in the 2004 and 2008 Olympics before gaining his United States citizenship in 2009.


He recalled racing Dale Oen, who began swimming at 4, for the first time in 2001, and over the years, they kept in touch. Their last communication, he said, came on Facebook before Dale Oen traveled to Arizona for the swim camp.


“He was really into nature and photography,” Alexandrov said. “He’d show me pictures he had taken of these crazy mountains in Norway.”


On his Twitter page Monday, Dale Oen, who lived in Bergen, said he was looking forward to going back home: “2 days left of our camp up here in Flagstaff, then it’s back to the most beautiful city in Norway.”


Dale Oen, who would have turned 27 this month, is survived by his parents, Mona Lillian Dale and Ingolf Oen, and an older brother.


Kitajima, who edged Dale Oen by 29 hundredths of a second in the 100 breaststroke in Beijing on his way to winning his second Olympic 100-200 double, said in a statement: “I was looking forward to racing against him at the Olympics. I’m sure he was looking forward to the Olympics, too. I still can’t believe he’s passed away.”


It was also hard for Gangloff to fathom that his target, the standard he has been chasing in his daily workouts, is gone.


“I was just shocked when I received the news,” Gangloff said. “It’s almost like it’s not real. You don’t expect that to happen to a person who’s at the peak of their physical condition.”


Gangloff, 29, a married father of a 2-year-old, has made the Olympics his focus. But he was having a hard time concentrating on swimming Tuesday.


“I think something like this puts everything in perspective,” he said. “All I could think about all morning as I played with my daughter is what his parents, his family and his country are going through right now. It’s heartbreaking.”