Texas couple dies of mysterious illness during 'amazing vacation' in Fiji, family says Authorities notified the couple's family a day before they were to return home.

A young Texas couple died of a mysterious illness while enjoying an "amazing vacation" in Fiji, according to their family.

Loved ones of David and Michelle Paul said they were in a state of disbelief on Monday after authorities on Fiji’s main island called to notify them of the couple’s sudden death.

It’s unclear how or exactly when the couple died, but Tracey Calanog, sister-in-law to the couple through her husband, who is Michelle Paul's brother, said family members got the devastating phone call the day before the couple were scheduled to return home.

A State Department official confirmed their deaths in a statement to ABC News on Monday, but it could not offer details on the situation. The department said it is monitoring the on-going local investigation.

"We offer our sincerest condolences to the family on their loss. We are closely monitoring local authorities' investigation into the cause of death," the spokesperson said. "We stand ready to provide all appropriate consular assistance."

Calanog told ABC affiliate WFAA that early indications suggests that the couple may have contracted an unknown virus -- one strong enough to debilitate two healthy, young people.

"They knew something was going on in their body and were able to get to the hospital," Calanog said Monday.

It appears as though Michelle Paul died first. Hospital workers told the family that they had planned to transport David Paul, an Air Force veteran, to Australia for further treatment, but he died before that could happen, according to the family.

Calanog described David Paul as a "very physically fit" and athletic person. The couple shared a 2-year-old son together and had spent a long time planning their long-awaited trip to the remote island.

"They were planning an amazing vacation," Calanog said..

She said the couple's remains cannot be returned to the United States until a cause of death is determined.

When reached by phone, the U.S. Embassy in Fiji told WFAA that it not comment on the Pauls' death specifically, but it noted that the government there had been warning of a dangerous flu season that is just hitting the region.

ABC News' Conor Finnegan contributed to this report.