The FBI may be considering a criminal investigation into the minor step-sibling of a young woman who was discovered dead aboard a cruise ship during a family holiday, according to fresh court documents.

Anna Kepner, 18, was travelling on a six-day Carnival cruise with her family when she was pronounced dead in international waters between Mexico and Florida on November 7.

A Daily Mail report claimed her body had been wrapped in a blanket, covered with life jackets, and hidden beneath her cabin bed before being discovered by a cleaner.

Circumstances surrounding the bubbly teenager's death remain undisclosed, though Florida Today reported that Miami-Dade medical examiner's office has conducted a post-mortem examination.

The Daily Mail reported that Anna initially complained of feeling unwell on the evening of November 6 and returned to her cabin.

The following day, her father Christopher Kepner, 41, and other relatives began scouring the vessel for her after she failed to appear for breakfast.

The revelation emerges in an emergency motion filed in divorce proceedings involving Kepner's stepmother, Shauntel Hudson.

In the motion - which requests rescheduling a hearing in the case - Hudson's solicitors state she has been informed by FBI investigators that "a criminal case may be initiated" against one of her minor children, CBS reported. "An extremely sensitive and severe circumstance has arisen wherein the Respondent/Mother will not be able to testify at the hearing at this time," the filing states.

Hudson's motion requests that matters concerning custody and access to the two minor children she shares with her former husband, Thomas Hudson, be delayed until the "criminal investigation has been concluded," reports Daily Mail.

"Any testimony the respondent may give, either written or oral, could be prejudicial to her or her adolescent child in this pending criminal investigation," it says.

Investigators are now examining surveillance footage from the vessel and analysing electronic swipe card data that may reveal who accessed the teenager's cabin, a law enforcement source told CBS.

They are also scrutinising Kepner's mobile phone records.

The FBI and ship security are also conducting interviews with other passengers, crew members, and Kepner's family.

Carnival Cruise Line issued a statement confirming that it is cooperating with the federal investigation.

The FBI is overseeing this case since Kepner, a U.S. citizen and resident of Titusville, Florida, died in international waters.

Carnival also said the FBI has told it there is no related threat to safety aboard the ship on its current voyage. Kepner, a high school cheerleader and gymnast, had aspirations of joining the military.

She was due to graduate in May from Temple Christian School in Titusville. Her father revealed to the Daily Mail that Kepner had ambitions to join the canine unit of the Titusville Police Department, with the aim of safeguarding her friends, family, and community.