A 16-year-old Florida boy has been charged as an adult with murder and aggravated sexual abuse in the death of his 18-year-old stepsister during a family cruise vacation, the U.S. Justice Department announced Monday.

The teenager, identified by the government as T.H., was initially charged as a juvenile on February 2, but U.S. District Judge Beth Bloom ordered that he would be prosecuted as an adult. The case had been sealed until the judge's decision was made public.

Anna Kepner was traveling on the Carnival Horizon ship in November with her family when her body was found concealed under a bed in a room she was sharing with two other teenagers, including her stepbrother. The ship was scheduled to return to Florida when the discovery was made on November 6.

The cause of Kepner's death was determined to be mechanical asphyxia, which occurs when an object or physical force prevents someone from breathing. Kepner was a high school cheerleader at Temple Christian School in Titusville, Florida, located about 40 miles east of Orlando.

The case is being prosecuted in federal court rather than state court because Kepner's death occurred in international waters, according to legal experts. Prosecutions of teenagers in federal court are extremely rare. The defendant appeared at the Miami courthouse on February 6, wearing a ball cap and hoodie that concealed his face.

At Kepner's memorial service in November, family members encouraged attendees to wear bright colors instead of traditional black "in honor of Anna's bright and beautiful soul."