A 16-year-old boy has been charged with murder and aggravated sexual abuse in the death of his stepsister during a family cruise vacation, the Department of Justice announced Monday.

Anna Kepner, 18, died on the Carnival Cruise ship Horizon in November while the ship was in international waters and heading to Miami, prosecutors said.

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The DOJ alleged the stepbrother “sexually assaulted and intentionally killed” Kepner. The Florida high school senior died from mechanical asphyxiation, officials said.

The boy was initially charged as a juvenile, prosecutors said. He’s now been indicted as an adult by a federal grand jury on charges of first-degree murder and aggravated sexual abuse.

Anna Kepner’s father, Chris Kepner, is married to the suspect’s mother, Shauntel Kepner.

Chris and Shauntel Kepner said in a statement Monday, “Our family is devastated by the loss of Anna and continues to grieve this unimaginable tragedy.”

The couple said the 16-year-old suspect has not yet been taken into custody, which they said has left them “deeply concerned and troubled.”

“We believe accountability is critical and are seeking clarity on this matter,” they said, adding, “We are placing our trust in the justice system to pursue the truth with care and integrity.”

“This situation is deeply painful and complex for our entire family,” the Kepners said. “Anna was deeply loved, and we remain committed to honoring her life and memory every day.”