Cynthia-Ann Day and Nathaniel Edward Holmes were found dead at a Dominican Republic Resort

Miranda Schaup-Werner had just checked into her Bahia Grand Principe hotel, in the Dominican Republic town of San Pedro de Macoris, and was taking pictures from her room balcony last Saturday when she started to feel ill. Less than two hours later, she was dead, local authorities said.

The 41-year-old Pennsylvania woman is the third American known to have died suddenly and under mysterious circumstances at two sister resorts in the Caribbean island nation last week, according to local authorities and the U.S. State Department.

Cynthia-Ann Day, 49 and Nathaniel Edward Holmes, 63, both of Prince George's County, Maryland, were found dead inside their room at the Bahia Grand Principe Playa Romana on May 30. Relatives had become suspicious after they didn't check out from the resort, located about 60 miles from the tourist-heavy Punta Cana area. The resorts, and the adjacent Bahia Grand Principe Bouganville, where Schaup-Werner was staying, are adjacent to one another on the island's southern coast.

The Dominican Republic's National Police is investigating all three deaths and awaiting toxicology results, officials said. Initial autopsy results for Day and Holmes showed they died of pulmonary edema and respiratory failure, police said. Investigators said they also found high blood pressure medication and three prescription pill bottles in the room, one of which contained 5 mg doses of the painkiller Oxycodone.

Jay McDonald, a spokesman for the Schaup-Werner family, told The Washington Post that Dominican police also indicated that his sister-in-law died of pulmonary edema and respiratory failure. The family declined to comment further. Hotel officials said in a statement on Wednesday that the woman died of a heart attack and that her husband, Daniel Frank Werner, confirmed to them she had a history of heart conditions.

Local authorities did not initially run toxicology tests for Schaup-Werner because there were no signs of violence, said Ramon Brito, a spokesman for the National Police's special tourism unit. But after the Maryland couple was found dead, investigators ordered a set of tests to determine whether anything the three Americans consumed may have led to their deaths, Brito said.

Dominican authorities have not released autopsy results for Schaup-Werner, who was visiting the country with her husband to celebrate her 10th wedding anniversary. They declined to confirm or clarify any details about the woman's death until the investigation was complete.

Hotel officials said their medical staff team responded to Schaup-Werner in her room and began treating her immediately. Before they could take her to the hospital, she was pronounced dead.

"During the event and in the days that followed we provided our complete support to Mr. Werner in collaboration with local authorities and the U.S. Embassy," the statement from Bahia Principe Hotel and Resorts said. "We once again express our condolences to Mr. Werner and his family and friends on the passing of Mrs. Schaup-Werner."

Holmes, of Temple Hills, and Day, of Upper Marlboro, were found five days later. An autopsy showed that both died when their lungs filled with fluid, leading to respiratory failure, according to a news release from the Dominican Republic's National Police.

There were no signs of violence, according to Dominican officials.

The couple had posted photos of themselves on Facebook enjoying time on the beach, wading in the turquoise blue waters of the Caribbean, riding all-terrain vehicles and cruising on a boat. On May 26, Holmes posted: "Can somebody please loan me $250,000 bcuz I don't want to come home!!!!!"

More than 2 million North American tourists flock to the Dominican Republic every year. But after an attack on a Delaware woman inside her resort near Punta Cana in mid-April, the State Department alerted travelers to exercise "increased caution in the Dominican Republic due to crime."