A teenager charged with sexually assaulting and killing his 18-year-old stepsister on a Carnival Cruise ship turned himself over to the custody of federal authorities on Monday after a judge reversed his decision on pretrial release now that the teen is charged as an adult.

The US attorney’s office in Miami confirmed that Timothy Hudson is in custody. US magistrate Judge Edwin Torres filed the order to revoke Timothy’s pretrial release last Wednesday, but the order was sealed until Monday afternoon. The order stated that Hudson should surrender to US marshals at the federal courthouse in Tampa Monday morning.

The judge had ruled in February that the 16-year-old could live with an uncle and be electronically monitored. But after the case was transferred to adult court in April, prosecutors wanted Timothy in custody.

The judge ultimately agreed that the issue of Timothy’s pretrial detention should be treated as if he was an adult, though Timothy will be held in an approved juvenile facility.

“The government has established, by clear and convincing evidence, that no condition or combination of conditions of release will reasonably assure the safety of the community going forward,” Torres wrote in his order.

Timothy has pleaded not guilty to charges of first-degree murder and aggravated sexual abuse in the death of Anna Kepner. His federal public defenders have declined to comment on the charges.

Minors are rarely prosecuted in federal court, and their names are usually withheld. But this case landed in federal court because Kepner apparently died in international waters, outside any state’s jurisdiction.

And Timothy’s name was released because he was charged with murder as if he were an adult.

Kepner had been traveling on the Carnival Horizon ship in November with her family, including Timothy. Before the ship was scheduled to return to Florida, her body was found concealed under a bed in a room she was sharing with Hudson and another teen, a criminal complaint said.

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

Attorneys for the prosecution and the defense made arguments for and against Timothy being jailed until his trial during a 27 May hearing. But Torres said he wanted to speak with the marshals service about the logistics of detaining Hudson in central Florida, closer to his family, rather than south Florida, where the trial is taking place. Hudson walked out of the courthouse after that hearing.

Alejandra Lopez, an assistant US attorney, argued at the hearing that the crimes Timothy is accused of are so serious that the court shouldn’t risk another violent attack. An autopsy determined that Kepner had been pinned down and forcibly raped, the prosecutors said. She also noted that it likely took three to five minutes for Hudson to allegedly strangle Kepner until she was dead.

The prosecutor also argued that Timothy was a much greater flight risk because he now faces a possible life sentence if convicted of the adult charges. As a juvenile, he would have been released at age 21, regardless of what counts he was found delinquent on.

Evan Kuhl of the federal public defender’s office told the judge during the hearing that Timothy has abided by the conditions of his release for months without issue.

Kepner’s father, Christopher Kepner, previously released a statement, saying the family was placing “trust in the justice system to pursue the truth with care and integrity”.

“The situation is deeply painful and complex for the entire family,” Kepner said.

Anna Kepner was a high school cheerleader at Temple christian school in Titusville, Florida, about 40 miles (65km) 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”.