Meanwhile, the FBI told ABC News it is offering resources to Guatemalan authorities to help in the search. The U.S. State Department has been in touch with local officials and is monitoring the investigation.
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"My dad, he ... I know this sounds terrible but he keeps his phone next to him at night," Nancy's sister Nicky Ng said in an emotional interview. "He's hoping for a ransom call, you know, something that will prove that Nancy might still be alive and that she's out there, that she's waiting for us to find her."
According to Ng's family, the last known video of her has emerged and was shared with searchers, showing her waving at the camera while kayaking on Oct. 19, just before she vanished.
Her family says Ng went on the kayaking excursion in the remote area of Lake Atitlán but she never returned.
According to a statement Guatemalan prosecutor's office provided to ABC News, Ng was traveling with a U.S.-based tour group. She went out on the kayaking trip and, according to a woman she was with, they advanced "approximately 2 kilometers deeper into the lake, where the missing woman tells" the witness that "she wants to swim and that is when she drowns," the statement said.
Why it took that woman and the tour group 24 hours to report the incident to authorities remains a mystery.
Family searching for answers after SoCal woman disappears in Guatemala
Local fire and rescue officials told ABC News that there were reports that Ng was last seen about half-a-mile away from the shore. Her kayak was later found, but no sign of Ng, a fitness enthusiast.
Her family says Ng works with special-need children in the Alhambra Unified School District. They describe her as a person who is loving and full of life.
"Please help us bring Nancy home," her mother, Stephanie Li, said in an interview.
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The family has hired private search team led by Black Wolf Helicopters owner Chris Sharpe.
"We've searched 99% of the lake -- that's was with helicopters, drones, boat crews," Sharpe said. The terrain, coupled with a lack of information, has complicated search efforts, Sharpe said.
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Ng's family said this was her second year going on the retreat. The family shared video that apparently shows Ng hours before she vanished.
"There are people that witnessed what happened, within the group, that haven't come forward," Nicky Ng said. "We're racking our brain as to why they wouldn't want to come forward and help, if nothing nefarious happened."