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FORT WORTH, Texas — Friends and family of David and Michelle Paul are struggling to imagine what could have killed the apparently healthy, young Texas couple while they vacationed recently in Fiji.

The trip was a getaway the Fort Worth parents try to make annually, just the two of them, according to WFAA.

One day before they were due back in the United States, however, relatives got a call that something had happened.

“They were planning an amazing vacation,” Tracey Calanog, Michelle Paul’s sister-in-law, said. “They knew something was going on in their body and were able to get to the hospital.”

While their cause of death is still not clear, family members told the news outlet they suspect it may have been some type of deadly virus, and possibly pneumonia.

Relatives told WFAA that David, an Air Force veteran, and Michelle, an athlete in school, were in great shape.

Michelle is believed to have died first after they got sick some time between May 22 and 25. There were reportedly plans to transport David to an Australian hospital, but he did not survive long enough.

The couple leave behind a 2-year-old son and David’s daughter from a prior relationship, WFAA reports.

The cause of their deaths is still under investigation.