A Texas couple vacationing in Fiji died from an unknown illness, a U.S. Department of State spokesperson said Monday.

Michelle and David Paul, from Fort Worth, arrived in Fiji around May 22 after dropping off their 2-year-old son, Ayden, with Michelle's parents, her father told NBC News.

Some time after they arrived, Michelle Paul called her father, Marc Calanog, to tell him she and her husband were experiencing vomiting and diarrhea, and their hands were numb. They went to a clinic and then to a hospital, where they were treated and then released. Calanog said David Paul was given IV bags to treat his symptoms, but Michelle Paul was not.

Then, Calanog learned that his son-in-law was in critical condition and was supposed to be transferred to a hospital in Australia, "but he never made it."

On May 25, the day before the couple was supposed to leave Fiji to return home, Michelle Paul's family received a call that she had died, her sister-in-law, Tracey Calanog, told NBC News.

Michelle and David Paul died within days of each other despite medical care, Fiji's Ministry of Health and Medical Services said in a statement, according to The Associated Press. The ministry said public health measures were put in place but did not specify what those were.

"Influenza has been ruled out, and at this stage we do not believe there is any risk to the public," the ministry of health and medical services said in a statement Wednesday."It would be premature to speculate further on the cause of death until the investigation is complete."

A statement from a Marriott spokesman said the couple was staying at Sheraton Denarau Villas on the coast, but each died "at the local medical facility."

"We have no reason to believe there are any health risks associated with the hotel," the spokesman said.

"We are deeply saddened by the death of two guests, one of whom was an associate of our company," the Marriott statement said.

Tracey Calanog said Michelle started working for Marriott several months ago and that David worked for aerospace and defense company Lockheed Martin.

The State Department spokesperson said they are monitoring local authorities' investigation into the deaths.

The U.S. Embassy in Fiji said in a statement Tuesday that it had been working with the Fiji government, police, and health officials for more than a week to determine how the couple died. A statement from the Centers for Disease Control and Prevention said officials there were working with Fiji officials who were sending specimens to CDC labs for testing in an effort to find the causes of death.

Tracey Calanog said both Michelle Paul, an athlete who swam competitively in college, and David Paul, an Air Force veteran, were in good health before leaving for Fiji.

"Nobody's telling me what was the cause of the death," Marc Calanog, Michelle Paul's father, said. "And that's very hard for me."

He said the couple had been anticipating the trip for a while and were excited when they left. He didn't worry about their safety because they traveled often and had the proper immunizations, as far as he knew.

He said he has been telling friends that "'our emotion is controlled because we believe my daughter and son-in-law is in a better place than us. They're ahead of schedule than me.'"

In reality, though, he doesn't feel so in control. "But I keep telling my wife we got to stay strong. Because we have a new job. Ayden. We have to take care of Ayden," Calanog said.

In addition to the 2-year-old, David Paul had three other children from previous relationships, his sister-in-law said.

The Fiji Ministry of Health and Medical Services said in Wednesday’s statement it was monitoring what it said was a "small number of staff and health workers who came into close contact with the deceased couple," and that all of those people are currently well.

The ministry also said it is coordinating with the U.S. Embassy in Fiji, the Fiji Police Force's forensics department, the World Health Organization and the CDC to determine the cause of death.