You get a few assorted clues among the useless bits of chatter that the hobos give you. One of them claims to have heard "human voices" and "lots of yelling". Others implicate the local monsters; one of these claims to have seen some gnolls leaving the area after the murder, while another claims that murlocs had been "casing the joint for days, maybe months" before killing the Furlbrows, only to leave when Laine and the others arrived.
Report your findings to Laine. Maybe he can make some sense of this. Sell your findings to someone who'd be interested in incriminating the humans.