MIAMI (AP) — A 16-year-old boy has been charged with murder and aggravated sexual abuse in Florida in the death of his 18-year-old stepsister on a Carnival Cruise ship, the U.S. Justice Department said Monday.

The teen, identified by the government as T.H., was initially charged as a juvenile on Feb. 2, but the case was sealed until U.S. District Judge Beth Bloom ordered that he would be prosecuted as an adult, the department said.

An email and voicemail seeking comment from T.H.'s lawyer about the indictment were not immediately returned.

Anna Kepner had been traveling on the Carnival Horizon ship in November with her family. Before the ship was scheduled to return to Florida, her body was found concealed under a bed in a room she was sharing with two other teens, including the younger stepbrother.

The cause of Kepner's Nov. 6 death was determined to be mechanical asphyxia, which is when an object or physical force stops someone from breathing.

Kepner was a high school cheerleader at Temple Christian School in Titusville, Florida, some 40 miles (65 kilometers) east of Orlando. At her memorial service in November, family members encouraged people to wear bright colors instead of the traditional black “in honor of Anna’s bright and beautiful soul.”

Teens prosecuted in federal court are extremely rare. T.H. was seen at the Miami courthouse on Feb. 6, wearing a ball cap and a hoodie pulled tightly around his face. But his status at that time was not fully known because his age barred public disclosures by his lawyer, the government or the court.

Experts believe the case is in federal court, and not a state court where teens are commonly prosecuted, because Kepner died in international waters.