ένα is used with neuter nouns, ένας with masculine nouns.
Masculine nouns usually end in one of "-ος -ης -ας (-ες)" in the dictionary form, neuter ones usually in one of "-ο -ι -μα".
And the feminine form of the indefinite article/number "one" is μια or μία (μια more often for "a, an", μία more often for "one").
Feminine nouns usually end in one of "-α -η".
Unless I'm mistaken ένας is used here but not with other professions. Is that because officer is viewed as a rank (within another profession)?
I wouldn't say so. Both are equally correct, and the translation without the article is included in the alternatives. ^.^