Police in Albuquerque, NM tell us the late porn star's official cause and manner of death is an accidental OD ... though it's unclear what specific substances were involved.

As we first reported ... cops in New Mexico considered Sophia's death "suspicious" and "unique" after she was found dead March 1 in her home.

During the death investigation -- which at one point included a homicide detective -- Sophia's mother told police her daughter drank heavily at times and was known to have suicidal thoughts.

Still, cops say Sophia's body showed no signs of trauma ... as they've now officially classified her fatal OD as accidental.