Anna Kepner's Carnival Cruise Ship Death Ruled Homicide

Anna Kepner was murdered on a Carnival Cruise ship ... at least that's the conclusion reached on her death certificate.

The document, obtained by ABC News, says Anna died as a result of "mechanical asphyxia" on November 6, and it notes the 18-year-old "was mechanically asphyxiated by other person(s)."

As you know ... FBI agents found two bruises on the side of Anna's neck, and she was thought to have died from a "bar hold" -- an arm placed across the neck.

Preliminary information indicates there were no drugs or alcohol in Anna's system and no signs of sexual assault ... but autopsy and toxicology results have yet to be released.

Anna was reportedly found by a cabin steward on November 8 -- wrapped in a blanket and tucked under a bed -- during a Caribbean cruise she was on with her father, Christopher Kepner, her stepmom Shauntel Hudson, and Hudson’s kids.

It's pretty unsettling ... a recent report claimed Anna's stepbrother allegedly had a weird infatuation with the teen cheerleader ... allegedly climbing on top of her in bed once, according to Steven Westin, the father of Anna's ex-boyfriend.

Anna's grandparents sat down on camera for a new interview where they claimed Anna and her 16-year-old stepbrother were "two peas in a pod." They said authorities told them the stepbrother is considered a suspect in the case, but they have faith in his innocence.