Annie Korkki and Robin Korkki were found dead in their luxury hotel room in Seychelles on Sept. 22 – as authorities work to find out what happened

Credit: Facebook

Two American sisters found unresponsive last week at a luxury resort while vacationing abroad both died of excess fluid in their lungs, which impaired their breathing, according to a report obtained by PEOPLE.

The autopsy results, released Friday, show that Annie Marie Korkki, 37, died of acute pulmonary and cerebral edema and sister Robin Marie Korkki, 42, died from acute pulmonary edema, according to a report released by the office of the Seychelles Minister of Travel and Culture.

Toxicology tests are being conducted, but results will not be ready for weeks, police spokesman Jean Toussaint told the Seychelles News Agency. Police said they found undisclosed prescription medication in the sisters’ room.

Edema is the swelling of tissue brought on by a build-up of fluid. Pulmonary edema restricts the flow of oxygen into the body through the lungs, due to the fluid.

A common cause of pulmonary edema is connected to heart problems, and in its acute form comes on swiftly – though pulmonary edema can also be caused by blood clots, near-drowning, reaction to certain drugs and viral infections, among other causes.

Cerebral edema, or swelling of the brain, can be caused by physical trauma, infection and strokes.

However, “no visible signs of injuries were found on the [sisters’] bodies,” according to the autopsy report.

The Korkkis were found unresponsive in the same bed on Sept. 22, days before they were set to leave their hotel on the Seychelles, an archipelago nation off the eastern coast of Africa, authorities said. The women arrived on Sept. 15 and were planning to leave on Sept. 24.

Annie worked at JPMorgan Chase in Denver and Robin was a financial trader in Chicago. They both attended high school in Minnesota.

Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter.

The two sisters, described as close, outgoing and adventurous, were staying at the $1,977-a-night Maia Luxury Resort and Spa after being on safari in Africa since mid-September, according to their Facebook pages.

According to an itinerary found at the hotel, the sisters had already visited Kenya, Tanzania and Zanzibar, police said.

Hotel staff say they saw the women consuming alcohol throughout the day before they were found dead, according to police. A butler helped the sisters to their room around 8:15 p.m.

The butler returned to the room at around 8:30 a.m. the next day and didn’t see signs that they were awake, police said. When the butler returned at 11 a.m. and the women didn’t answer the door, hotel staff entered the room and found them unresponsive, according to the Seychelles News Agency.

Messages left with Korkki family members and Seychelles police have not been immediately returned.

“We still don’t know how they died,” brother Chris Korkki told NBC News from his home in Minnesota on Thursday. “The U.S. Embassy has been incredibly helpful, but there are more questions than answers.”

The women’s mother and another brother have flown to the Seychelles to try to find out what happened, Chris said.