The family of a Denver man who was visiting the Dominican Republic with his daughter says he has died in a local hospital after he became critically ill this week.

Khalid Adkins, who was staying in Punta Cana, tried to return home Tuesday, relatives told Fox News, but the crew on the flight removed him after seeing how ill he was. A Denver NBC affiliate, 9 News, reported that the daughter with whom he was vacationing said he started complaining about feeling unwell on Sunday, and got progressively worse.

“We found out this morning that he passed away last night! I’m at a loss for words, we have no explanation of what happened, all they will say is he got sick,” the family wrote Wednesday on a GoFundMe page they set up to raise funds to rush his body's return to the United States.

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The U.S. State Department confirmed Adkins' death to Fox News Thursday.

Many details were unclear Thursday morning, including whether the father, who was 46, and daughter, Mia, had stayed at any of the resorts where others U.S. tourists were when they died or got very ill and died shortly after.

Mia told 9 News that she left on Sunday, before her father, who had planned to stay some extra days, because she was due back at work.

She said that at first, he complained of pain in his leg, and said there was a bump that looked like an insect bite on the area. He nonetheless drove her to the airport.

"That’s the last time I saw my dad," she said.

When he felt much worse the next day, his family got him a new reservation to get back to the U.S. as quickly as possible and go to a doctor here. But when he got on the plane, he began exhibiting severe symptoms, including vomiting and sweating profusely, the relatives said. He was taken to a hospital, where they noted his shortness of breath and kidney failure.

Relatives, who last heard from Adkin when he told them he'd been taken off the flight and was going to the hospital, said they had trouble getting information from the medical staff, in part because of language barriers. They said that the hospital made little attempt to keep in touch with them to let them know what was happening and that they were not alerted about his death until the next morning -- and only after they called several times to get an update on his condition.

"We're still trying to figure out what is going on," his sister-in-law, Marla Strick told Fox News on Thursday. "We were trying to desperately to get help, get information, so we that we could get him here. I don't think we'll ever know everything that happened."

Adkins' death comes as the FBI is investigating the deaths of at least three U.S. tourists who died within five days in May in their rooms at the Bahia Principe resort complex in La Romana.

The three—including a Maryland couple found dead in their room —are part of a rash of U.S. tourists whose deaths in the past year have been made public by relatives who say they have doubts about the determination by Dominican authorities that they perished due to natural causes.

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The relatives have told reporters similar stories of being stonewalled by authorities and resort staff there when they sought information about the tourists’ deaths.

"When people get sick there, it should be easier for families to get information," Strick told Fox News. "It shouldn't be this hard."

The daughter, Mia, told 9 News: "It's been hard, not being able to get a hold of them, or them miscommunicating, or simply not knowing information."

In most of the deaths, a heart attack was listed as the cause of death.

Adkin's family told different media outlets that they have not been given a diagnosis about what killed him. Authorities are conducting an autopsy, 9 News reported. Adkin's family said he was not sick before the vacation. Relatives did note that Akin had a kidney transplant several years ago, though they said he was doing well since then.

Several of the nearly 12 U.S. tourists whose deaths became public fell ill after consuming a beverage from the room minibar, prompting Dominican authorities to conduct tests of the minibars, but also the resort food, pool, air conditioning, and vents, among other things.

Many tourists have reached out to the media with accounts of falling suddenly ill after having a drink or upon being exposed to smells that seemed to come from insecticides or cleaning substances.

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A Colorado couple, who say they became seriously ill at a Bahia Principe resort last year after being exposed to a strong smell in their room, has filed a lawsuit claiming they were poisoned.

Dominican officials told Fox News their tests for toxins or other elements at the resorts have come back negative.

They maintain the deaths are not mysterious and are, instead, rooted in pre-existing medical conditions. They stress their country is safe for tourists. The Dominican Republic hosts some 6 million tourists a year, of which more than 2 million are from the U.S.

Dominican and U.S. authorities say that about 15 U.S. tourists die in the Caribbean vacation spot each year, and that compared to the large contingency that visits annually, it does not rise to the level of rendering it an unsafe place. The statistics, however, exclude the category of deaths that are making headlines -- those attributed to natural causes by Dominican authorities. The data accessible to the public covers only non-natural deaths, such as drownings, homicides and car accidents.

Neither government has provided the number of natural cause deaths to Fox News, despite repeated requests.