Missing with dissociative fugue: Hannah Upp's mystery disappearance

In 2008, a teacher survived a three week disappearance in New York that she had no memory of. She vanished again in the U.S. Virgin Islands 9 years later - and never came home.
6:27 | 05/16/19

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Transcript for Missing with dissociative fugue: Hannah Upp's mystery disappearance
Reporter: It was a category five hurricane. Erma. A storm unlike any the caribbean paradise of St. Thomas had seen before. It was in some ways a perfect storm of chaos. If someone's going to vanish from the island without any trace it could have happened then. Reporter: And that's exactly what happened. 32-year-old Hannah, a beloved teacher at a local montessori school had ridden out the storm only to disappear. No one had seen her, and I went to her house and she wasn't there. Her car wasn't there. She drove down to sapphire beach, inside the car were all of her personal items. There was no sign of foul play. She simply vanished. Reporter: What many people didn't know, this was not the first time Hannah had disappeared. I did not know about her previous disappearances. Reporter: Almost ten years before, Hannah went missing in New York City. She was found barely alive, floating face down in the new York harbor. No memory of where she'd been for the past three weeks. Doctors diagnosed her with dissociative fuge. It means you have a period where suddenly you've lost your auto biographical memories. The memories that define you. Reporter: This rare disorder is sometimes known at Jason Bourne syndrome. What's your name? Reporter: From the wildly popular movies about a CIA assassin who doesn't know who he is. But remeers his training. Elizabeth vargas and A&E's the untold story "Vanished in paradise" retraces Hannah's life and the days before she disappeared. She grew up the daughter of two methodist ministers in Oregon, and at 23 became a public schoolteacher in New York City. Everybody describes Hannah as a bright person, engaging, open. Reporter: In 2008, she left her New York City apartment to go running and went missing. New York City police thought, we're dealing with a terrible crime here. They were looking for a dead body. Hannah was last seen at her Hamilton heights apartment last Friday. Reporter: She became a high-profile missing persons case. She seemed to have vanished in thin air. Someone has caught a glimpse of a missing teacher. Ten days later she pops up at an apple store of all places, logging in on her own id and security footage shows her being approached by a young man who knows her from school saying, Hannah, you're Hannah, right? Aren't you missing? And she says, no. I'm not Hannah. And walks away. So a person who has this, to people on the outside, may look completely normal. They're doing things. They know how to do things. They understand the world at large. They don't appear to be distressed. They're just in an altered state. Reporter: Later, she was spotted using her own gym pass. Investigators were perplexed. Authorities couldn't track her down until she was pulled from the water by crew members of the Staten Island ferry. Hannah's mother Barbara remembers vividly the moment she found out her daughter was still alive. Early afternoon when the phone rang, this is a nurse at the emergency room. What did she say? She told me my daughter had been pulled out of the water, and she was alive. She was dehydrated. Badly sunburned and hypothermic, but when I first saw her, she had no idea what happened? What did she say? Mom, I can't remember what happened? She said I'm fine, but I'm scared. I don't know what happened. Reporter: The documentary has this recording of Hannah after she was rescued. She's talking to a reporter in a New York City coffee shop. It goes from like going from a run to being in an ambulance. For me, it was like ten minutes fast, but it was like three weeks. The hardest part is the period right after. Right. You feel shame, and you feel embarrassed. All things that I definitely felt. Reporter: After her fuge, Hannah moved to Maryland and there had another episode. Many people in her life believe she was looking to escape her past. Reporter: So in 2014, Hannah moved to the island oasis of St. Thomas. She is the center of the party. Reporter: She threw herself into island life, embracing a new set of friends who shared her passion for Zumba. But the hurricane wreaked havoc on the island and some say on her, too. Everybody was stressed out and she was certainly stressed out. And certainly, the day before Hannah disappears, people notice something was off. Be almost in a trans. Disassociated fuge appears in people who suffer a tremendous trauma. It may be a trauma in their past or a recent trauma, like living through a natural disaster. Reporter: But Hannah's mother insists that for her daughter the episodes do not have clear triggers. I think what's mysterious is she loved her life. She was a young woman who definitely loved her life. That doesn't mean she couldn't have been struggling with something deep and dark inside of herself. Reporter: It's been almost two years since anyone's seen Hannah. Her mother has become a fixture on St. Thomas. She thinks it's possible Nan Hannah is still alive and suffering from another fuge. There are documented cases of fuge states lasting as long as four years. Yeah, and that helps me to keep on going. It's never been an option to give up. Reporter: And hopes thence to where her daughter is can be found here. Do you think you'll ever know for sure, Barbara, what I can only tell you that that hope is persistent, and many people join me in that hope. And you can watch the untold story "Vanished in paradise", tomorrow night on A&E. Up next, he as back with

This transcript has been automatically generated and may not be 100% accurate.

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