Concerning the articles
Orders : Investigate the drowning, apprehend the murderess, incarcerate or otherwise neutralise the threat
* Check back cases - have there been any previous incidents of drowning involving an unknown female?
* Question witnesses - was there anything unusual about the victim's reactions or actions? Did they notice anything unusual or noteworthy about the unknown female (tail or legs? greenish cast to skin? hair colour? singing?)
Working hypothesis - the unknown female was an aquatic sapient, likely recently moved to the area. (question locals about any newcomers or recent immigrants from Undersea)
Possible suspects :
Siren (Greek) Sirens are genetically inclined to be excited by the death of their human male partners - much like preying mantises ... BUT not all mermaids are Sirens. And not all Sirens give in to their genetic inclinations.
Characteristics: Sirensong is extremely erotic and irresistible to men; rarely do the males survive the encounter. They are related to Harpies and may have a secondary avian form; with the lower halves and wings of a bird (information suspect, rarely observed. Unlike Harpies, Sirens are incapable of flight and so rarely take this shape.) (may have to question local Harpie population as well, god help you)
Their power is strongest when the moon is full.
Nereid (Greek)Nereids are the attendents of Queen Thetis and variously claim to the be the daughters of Poseidon or Nereus (the shapeshifting "Old Man of the Sea"). They are cousins to the Harpies, Gorgons, Phorcids; some among them are Sirens. The are NOT inherently dangerous and can be kindly disposed toward mortals.
Rusalka (Slavic)Rusalki are the spirits of women betrayed by men who committed suicide by drowning. They seek revenge for their betrayal and use song magic to lure males to their death.
Characteristics: Rusalki are humanform, female, with translucent skin, occasionally have tails. They are also shapeshifters who can transform into water creatures ... INCLUDING HORSE!! While they must remain near water, they are not confined to it and may live on land. Fresh and saltwater spirits.
Merwoman (mid-Atlantic & Pacific)Merwomen tend to be more mature than the usual watersprite nor is their song as irresistible; they tend to rely more on appearance and conversation than magic. They do take human males as lovers, but are usually careful when they introduce them to their watery home. However - most merwomen are married and their cuckold mermen are often not kind to the mortal lover.
Characteristics : mature appearances, lovely, long hair, translucent skin, EXTREMELY buxom (breasts can be thrown over shoulders!!)
Lorelei Young woman, sings men to death but unlikely. Lorelei are freshwater spirits, not brine.
Nix (Scandanavian)Characteristics: freshwater spirit (species has two genders, not only female), seldom beautiful, with green hair and skin. NOT inherently murderous.
Selkie (Gaelic)Seal people (species has two genders, male and female)
Characteristics: Selkies are seals in the water, women on land. Unlikely for the Unsub, imho
Notes
- unknown female may claim diplomatic immunity, especially if she is one of the daughters of Poseidon - please note, ALL the Nereids claim to be royalty descended from Poseidon
- a recent immigrant from Undersea may not understand that humans aren't prey
- I like the Rusalka for this, especially if we can connect her with the attack at Badwolf