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)." Play video content

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 Caribbea

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