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(CNN) —  

It took less than five minutes for a weekend bachelor party in Costa Rica to turn into a tragedy that claimed the lives of four men, including the groom’s brother.

Ernesto Sierra, Jorge Caso, Sergio Lorenzo and Andres Denis were killed during a white-water rafting tour on the Naranjo River in Quepos on Saturday. The men, all Americans, were between 25 and 35 years old. A fifth person, a Costa Rican tour guide identified as Kevin Thompson Reid, also died, according to Costa Rican officials.

“What was meant to be a weekend to remember for 14 friends turned into a living nightmare,” said Anthony Castro, one of the group’s survivors, who started a GoFundMe page in honor of his friends.

Castro said the group had planned a number of activities during their trip to Costa Rica, which they were determined to make memorable for their longtime friend, Luis Beltran.

“We’ve all been friends for many years and we knew we wanted to do something special for our friend before his wedding.”

Castro said the bachelor party arrived at the river amid high winds and rain, which led the rafting tour guides to delay going out on the water for just under an hour.

After the guides gave the all clear, the group piled into their rafts.

“Within five minutes of being out on the river, all three rafts capsized, and everyone ended up in the water,” Castro said.

Thirteen of the 18 people pitched into the water were able to grab hold of the overturned rafts, Costa Rican officials say. Other members of the party were swept down the river.

Castro said the river’s “immense current” kept flipping the rafts as he and others tried to pull themselves out of the water. Eventually he was among those carried away, banging off rocks in the water.

Those carried downriver were wearing life jackets and helmets, but Castro said it was a battle to keep his head above water. “All of us struggled to stay above water, swallowing lots of it on the way down.”

Castro said he and some others were able to grab onto rocks or barriers in and around the river, but four of his friends, including the groom’s brother, were not among them.

“Luis, the man we hoped to celebrate all weekend, lost a brother and we all lost four great friends during this vacation which went horribly wrong,” Castro said.

The GoFundMe page Castro started will help pay for funeral expenses for his lost friends. He says any additional money raised will go to Lorenzo’s children.

Costa Rican President Carlos Alvarado Quesada said on his official Twitter account: “In the name of the government and people of Costa Rica I would like to express our deepest sorrow for the accident this afternoon at the Naranjo River in Quepos. I would like to thank all the emergency and security teams that worked diligently, and I have instructed authorities to help the victim’s families.”

CNN’s Claudia Dominguez, Tina Burnside and CNNe’s Djenane Villanueva contributed to this report.