Mystery surrounds the sudden deaths of a US couple holidaying in Fiji after local officials admitted they did not yet know how they died on the island almost two weeks ago.

Forth Worth couple, US Air Force veteran David Paul, 37 and his wife Michelle, 35, who worked for Marriott International, died within two days of each other.

Their two-year-old son remained behind in the US with relatives.

Mrs Paul’s father, Mark Calanog, told the Fort Worth Star-Telegram that he got a text message from his daughter on May 24 saying: “We are both going to doctor now. We have been throwing up for eight hours”.

“David also has diarrhoea. My hands are numb.”

Las Vegas-based Mr Calanog said he got a call 24 hours later on May 25 that Michelle was dead, and that David Paul was dead two days later.

Ms Paul’s sister-in-law Tracey Calanog told local TV station WFAA TV  the couple had been planning this “amazing vacation” and were scheduled to return home only 24 hours before they got sick.

A nurse who looked after the couple told the FijiSun News they were brought in on a buggy from their hotel to a Nadi medical centre early on May 25 with symptoms including vomiting, diarrhoea and weakness and were put on saline drips.

They both wanted to return to their hotel but Ms Paul collapsed in her room and was brought back to the centre. She died before reaching the clinic.

In shock, Mr Paul was admitted to Lautoka Hospital’s ICU where his condition deteriorated and he died on May 28.

The nurse, and a colleague, a police officer and two security guards experienced symptoms and were admitted to Nadi Hospital for several days before being discharged.

Influenza has been ruled out as a potential cause of death, according to local health authorities.

On June 4, the US Embassy in Suva issued a statement saying it was continuing to work closely with the Fiji government.

“For more than a week, the US Embassy has been closely working with the Fiji government, police, and health officials in their investigation into the tragic deaths of two US citizens while on holiday in Fiji,” it said.

“The embassy has facilitated contact between the US Centres for Disease Control and Fiji’s Ministry of Health to ascertain the causes of death and to assist in any related measures.

“At this point, we are not aware of any specific public-health threat to the community,” the statement said.

Mr Calanog said on June 4 he believed an infectious disease may have killed the pair, according to the New York Daily Post.

“I gave them instruction not to do any cremation until I’m satisfied … because I would like the whole world to know if they died of infectious diseases,” Mr Calanog told ABC News.

“The [Centres for Disease Control and Prevention] can confirm that, and tell Americans and other people that these are the issues you’re going to face if you have to travel to Fiji. Be aware.”

He sad his daughter was a “world traveler,” and wanted to take her husband to Fiji because he hadn’t travelled much.

“They are much in love and they just bought a house in Fort Worth. And they were enjoying it and this tragic thing happened,” he told America’s ABC News.

Fiji’s Ministry of Health and Medical Services said on Wednesday the investigation was ongoing.

It said staff and health workers who had been in contact with the couple were being monitored as a precaution, but that all are currently well.

The ministry said it’s also working with local police, the World Health Organisation and the US Centres for Disease Control and Prevention to determine the cause of death.

Mr Calanog told the paper his daughter and Mr Paul met five years ago in Hawaii and both loved beaches and snorkeling.

“I did not think it was so serious,” he said.

“I was stunned. I almost cried, but I didn’t cry because I was holding onto my emotions … I would say I was emotionally shocked.”

-with AAP