Anna Kepner was found dead in her cabin on the Carnival Horizon cruise ship, with the FBI and Miami-Dade medical examiner's office confirming homicide by strangulation.Her 16-year-old stepbrother, who was on the cruise with Anna and is currently the prime suspect, was briefly detained and released as the FBI investigation continues.Family members and acquaintances described past conflict and fear between Anna and her stepbrother, while Anna's grandparents publicly defended the stepbrother and the Carnival Cruise Line stated it is cooperating fully with the FBI.