(CNN) -- The family of the U.S. swim star who died over the weekend during a race in the United Arab Emirates mourned Monday as answers to questions surrounding his death remained murky.
Fran Crippen, 26, died Saturday during the last leg of the Marathon Swimming World Cup.
"The official word coming out of the UAE ... is that he died from drowning. So now we have to go and figure out exactly how that happened because obviously Fran was a swimmer who was in peak physical shape," the swimmer's sister, Maddy Crippen, told CNN.
Another athlete in the race told CNN the competition might not have been monitored closely enough.
"I may be mistaken, but I can't remember seeing any boats that were going by us to watch the athletes a little bit closer, especially the ones that were falling behind," said German swimmer Jan Wolfgarten.
"With the conditions that extreme, for future reference, there should be people watching what's going on with the athletes," Wolfgarten said.
Monitors on at least nine boats and four personal watercraft were watching the race, according to event organizers.
Wolfgarten said he thought Crippen "was in just as good a shape as anyone can be."
But both the water and the air were "really, really warm," Wolfgarten told CNN's "American Morning."
"The sun came straight down" during the last lap, just before noon, he said, suggesting it might be better to do the race in late afternoon next year.
Crippen's sister said she had heard from people who were there that her brother wasn't "charging" like he usually did when he swam.
"He wasn't himself as the race continued," she said.
U.S. Swimming Federation authorities said they expect to receive the body of the star open-water swimmer Monday.
"In respect for his memory," the International Swimming Federation (FINA) canceled the remaining heats in the UAE in the wake of Crippen's death, according the federation's website.
The loss of Crippen hit hard in suburban Philadelphia, Pennsylvania, where he grew up in a family of swimmers. Crippen was on target to achieve his goal of being an Olympian -- a dream cut short.
"We're all grieving. We're all in shock," said Richard Shoulberg, his longtime prep school coach.
Doctors determined that severe fatigue was behind Crippen's sudden death, UAE Swimming Federation Secretary Saeed Al Hamour told CNN on Sunday.
Earlier, however, the head of the water sports club in Fujairah, Ahmed Ibrahim, said that preliminary tests showed the swimmer suffered a heart attack.
The official report released by the UAE Swimming Federation made no mention of a heart attack.
Crippen died during the last leg of the Marathon Swimming World Cup in Fujairah, the International Swimming Federation said in a news release. The cause of death is under investigation, the federation said.
In a conversation with Shoulberg 12 hours before the race, Crippen said that the outside temperature was 100 degrees and that the water was 87 degrees.
Some have speculated the high temperatures may have contributed to the death.
However, Al Hamour denied any reports that Crippen's death may have been caused by water temperature.
"The competition was monitored and supervised by the International Swimming Federation. All security measures were taken care of as needed," Al Hamour said. "We've organized so far 14 competitions and championships and never had any death."
Swimming World magazine reported that Crippen fell unconscious during the event and deep-sea divers found him two hours later near the race's final buoy.
Crippen won bronze in the 10-kilometer event at the 2009 FINA World Championships and was the gold medalist in the same event at the 2007 Pan American Games, according to USA Swimming.
Monday, his family released a statement thanking Crippen's fans and friends for their support.
"Fran's life was filled with passion, whether it was swimming for the U.S. or cheering on his beloved Philadelphia Phillies. We take some solace knowing he passed while doing what he loved most, swimming," it read.