The sisters — 37-year-old Annie and 42-year-old Robin — died last week on an island in the Indian Ocean. The American siblings were vacationing at a high-end resort in Seychelles, where their bodies were reportedly discovered Sept. 22.

In the immediate aftermath of their deaths, few details were known.

Now, even after the release of autopsy results, the mystery surrounding their deaths persists.

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Police in the Republic of Seychelles released a report Friday showing that the women died from an excess of fluid in their lungs, according to NBC News. The autopsy report states that Robin Korkki died from acute pulmonary edema and that Annie Korkki died from acute pulmonary edema and cerebral edema, Seychelles police spokesman Jean Toussaint told the network.

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An edema is the result of swelling caused by an accumulation of fluid, according to the Mayo Clinic.

The autopsy indicted "no visible signs of injuries found on the bodies," Toussaint told NBC News.

Results of a toxicology report are pending, and Toussaint told NBC that those tests may provide more clues about what happened to the sisters at the $2,000-a-night Maia Luxury Resort and Spa.

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The Seychelles News Agency reported last week that Annie and Robin Korkki were "unresponsive" when they were found at the resort, according to police.

They were taken to a medical center, where the sisters were "certified dead" by a doctor, police spokesman Jean Toussaint said in a statement released to the news agency. Authorities conducted a preliminary investigation of the bodies, the statement noted, but didn't find any "signs of violence."

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"There were no marks on them whatsoever," Seychelles Tourism Minister Alain St. Ange told NBC News. "They had a good time in the day and then they went to their room."

Toussaint, the police spokesman, told Seychelles Nation that the sisters were drinking the night before and were helped to their villa by hotel personnel. A butler noticed that the villa's door was still locked the next morning. He alerted hotel personnel after the day continued without any movement in the room.

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"From the observation done by our police officers, there was no disturbance in the room and the two sisters were found unresponsive on the same bed," Toussaint told Seychelles Nation.

Seychelles is off the east coast of Africa. According to the Seychelles News Agency, the sisters were scheduled to leave Saturday, and had arrived Sept. 15.

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Chris Korkki told the AP that "his mother and brother have traveled to Seychelles for answers and to make arrangements to bring his sisters' bodies back to the U.S."

"At this point, the only details we know are the articles flying around online," Chris Korkki told the Star Tribune. "My mom has been talking with people from the U.S. Embassy. I don’t think they’ve provided her with any information."

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The Star Tribune's report continued:

The brother described his sisters as healthy and adventurous women who "definitely wanted to experience life to the fullest." He added, "Two things keep going through my mind: This isn't happening, and we just want answers."

In an email Thursday to The Washington Post, a State Department official referred questions about the investigation to local authorities.

"We are aware of the deaths of two U.S. citizens in the Seychelles last week," the email said. "We extend our deepest condolences to family and friends, and stand ready to provide all appropriate consular assistance."

The AP tried to check in with local authorities in Seychelles, as well as with the foreign ministry, but were unsuccessful in their attempts. NBC News also tried to get speak with those involved in the investigation, but their calls were not returned.

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"It's all very surreal," Chris Korkki told KARE, an NBC affiliate in the Minneapolis-St. Paul area. "We don't know very much. Our family is still very much in shock. We're devastated."