A graduation trip to the Dominican Republic for a group of Oklahoma teens took a turn earlier this month when part of the group became mysteriously ill during their stay in the Caribbean nation, where at least nine Americans have died while vacationing.

About 40 seniors, who just graduated from Deer Creek High School in Edmond, traveled to the Dominican Republic on June 8 to stay at the Hard Rock Hotel and Casino in Punta Cana, a parent told KOTV.

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The all-inclusive resort is the same location where two Americans have died and others have gotten sick.

The group ate at a Japanese restaurant on the resort and at least five people became ill, Liz McLaughlin, whose daughter is sick, said.

“We just don’t know what is happening," she told news station. "Is it the water? Is it the ice? Is it the food? Is it the food handling? Is it the pesticides? We have no idea what’s going on."

Bennett Hill, a student who became sick on the trip, told KOCO that on their second day in the Dominican Republic, he woke up sweating, freezing and cramping.

"We've been hooked up to IVs since we first got here with antibiotics, just getting hydrated," he said, adding that six other graduates were also hospitalized for symptoms. "Anti-nausea medicine, all this stuff because we were just so dehydrated."

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But according to the superintendent of Deer Creek Public Schools, most of the former students were fine and only those that ate at the restaurant became sick. As of Monday, most of those who traveled with the group were back in the U.S., according to The Oklahoman.

The popular Caribbean vacation destination has been grappling with a rash of deaths of U.S. tourists at various resorts. Families of the tourists said they were generally in good health.

Of the recent deaths that have become publicly known so far, Dominican investigators said five were caused by a heart attack. In the case of the other two — an engaged Maryland couple found dead on May 30 — a final report on the cause of death was pending.

The family of Jerry Curran, 78, from Bedford, Ohio, came forward last week to say their father passed away on Jan. 26, three days after arriving in the country with his wife.

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“He went to the Dominican Republic healthy and he just never came back,” Curran’s daughter Kellie Brown told WKYC last week. “I thought something’s not right. My father was a healthy 78-year-old, he took care of himself and I just didn’t think anything like this was possible, but then I started to hear other people’s stories.”

A ninth person, identified as Joseph Allen, 55, of Avenel, New Jersey, was found dead on his hotel room floor on Thursday morning, according to local reports. The State Department confirmed to Fox News that another American died in the Dominican Republic, but they did not provide an identity.

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In nearly all of them, officials said there were signs of pulmonary edema — a condition in which the lungs fill with fluid. The tourists ranged in age from 41 to 67.

The Dominican Ministry of Tourism has denounced what it has called an overreaction to what it characterized as coincidental.

Fox News' Frank Miles contributed to this report.