La Romana, Dominican Republic (CNN) Miranda Schaup-Werner, the first of three American tourists to die mysteriously within a week at a Dominican Republic resort, succumbed to a heart attack, the Caribbean island's attorney general said.

Authorities released details Thursday of the preliminary autopsy reports for the Pennsylvania woman as well as Maryland couple Nathaniel Holmes and Cynthia Day. While it remains unclear what may have caused their deaths, officials offered a glimpse into the conditions of their bodies.

The couple had internal bleeding, including in their pancreases. Holmes had an enlarged heart and cirrhosis of the liver -- both signs of significant pre-existing disease. Day also had fluid in her brain. The couple also had fluid in their lungs, Attorney General Jean Alain Rodriguez Sanchez's office said in a statement.

Authorities said they won't be able to provide more details on the causes of death until toxicology results are completed. Investigators found "several pill bottles" of three medications, including the prescription opioid oxycodone, in the couple's room, the attorney general's office said.

Schaup-Werner checked into the Bahia Principe Bouganville resort in La Romana on May 25 and excitedly took pictures in the room she shared with her husband, Dan Werner. The pair was celebrating a wedding anniversary.

She had a drink from the minibar and suddenly felt ill, family spokesman Jay McDonald told CNN affiliate WFMZ . Shortly afterward, she collapsed and died .

Five days later, Holmes, 63, and Day, 49, missed their scheduled checkout time at the Bahia Principe La Romana, a hotel located at the same resort. When hotel employees checked on them, they were dead, police said.

Health inspectors, including environmental health and epidemiology specialists, were inspecting the hotels Thursday and should have results Friday or Monday, said Carlos Suero, spokesman for the Ministry of Public Health.

Tourism Minister Francisco Garcia said Thursday that the Dominican Republic is safe and called the deaths "isolated incidents."

'We just want to understand this'

Hotel workers found Holmes and Day on May 30. They checked into the hotel May 25 as well.

"The bizarre issue of the same hotel and these things happening within days of each other and the complete unexpected nature of what happened to Miranda, we just want to understand this," McDonald told the affiliate

Cynthia Ann Day, Nathaniel Edward Holmes and Miranda Schaup-Werner

"At one point, she was sitting there happily smiling and taking pictures and the next moment she was in acute pain and called out for Dan and she collapsed."

Holmes' daughter said she wants the mystery of her father's death solved.

Taxi driver says victim appeared happy

A taxi driver who picked up Schaup-Werner and her husband from Santo Domingo Airport and dropped them off at the resort told CNN the couple appeared happy and tipped generously.

The driver, who did not want to be identified, said the trip took 40 minutes from the airport to the hotel. Once they arrived, he helped them with their luggage and left.

Days later, the driver said, he went to pick them up for their return trip back to the airport, and was told they'd checked out.

He found out Schaup-Werner had died from the news, he said.

Maryland couple took part in other activities

Holmes and Day were from Prince George's County, and were supposed to fly back home the day they were found.

Three days before they were found dead, they went on an excursion to Saona Island, according to Bahia Principe. The hotel chain believes the couple visited the capital because a bag from a Santo Domingo pharmacy was found in the room.

Holmes posted Facebook photos of him and Day enjoying their time in open waters.

"Boat ride of a lifetime!!!" one caption said.

Authorities give more details

In the case of Schaup-Werner, paramedics provided first aid, but she died in the room, according to Col. Frank Felix Duran Mejia of the Dominican Republic National Police. No violence was involved, the police official said.

As for Day and Holmes, police said an autopsy concluded the couple had respiratory failure and pulmonary edema, an abnormal buildup of fluid in the lungs.

Blood pressure medications were found in the room, along with three medications, including an opioid and an anti-inflammatory, according to police and the attorney general.

"The case of Mr. Holmes and Ms. Day remains under investigation by the authorities with the results of toxicology tests still pending," the resort operator said. "We disapprove of any conjecture on possible causes of death and urge all to respect the families while the investigation is ongoing."

Resort's information differs from officials

Bahia Principe Hotels & Resorts said it followed all security protocols. There are no indications of any connection between the two cases, it said.

The resort further said Schaup-Werner's husband indicated she had a history of heart conditions.

McDonald said she was treated for a heart condition 15 years ago but had not had more recent issues.

"She had been seemingly healthy. He was not aware of anything going on with her health," he told the affiliate.

Millions of Americans visit the Caribbean nation

Tourism is a key part of the nation's economy, and its coastal resorts are a popular among travelers.

The tourism industry made up more than 17% of the country's economy last year, according to the World Travel & Tourism Council.

In 2018, about 6.5 million tourists visited the Dominican Republic, more than any other Caribbean nation, according to the Caribbean Tourism Organization. The country attracted 2.2 million tourists from the United States, which is more than any other country in the region.

Forty-eight Americans died in the Dominican Republic between 2016 and 2018, according to statistics available on the US Department of State website. The statistics, which do not include deaths from natural causes, include deaths from drownings, homicide, suicide, vehicle or other accidents.

New York couple killed in a crash

The three deaths in May happened a few months after a car crash in the Dominican Republic killed two other Americans.

In March, a car carrying New York tourists Orlando Moore and Portia Ravenelle plunged into the Caribbean after the pair left for the Santo Domingo airport to catch a flight home, the nation's police said.

The body of Moore, 40, was found at sea, authorities said.

Fishermen found Ravenelle, 52, and took her to the hospital, but she died days later.

CORRECTION: The headline and story have been updated to reflect only Nathaniel Holmes and Cynthia Day had fluid in their lungs. Previous versions of this article also transposed Nathaniel Edward Holmes' first and middle names.

The US Department of State website, which reports 48 Americans died in the Dominican Republic between 2016 and 2018, does not account for those who may have died of natural causes.