Have Anita be the special expert brought in to solve the crime. That way the guard can refuse to do more than patrol saying "wait for Anita, its her case now." when she arrives the pcs have to earn her respect in order to get her helpful. you could have several Paths diverge from there: Anita and the guard are extremely hostile, anita and the guard are competitors to solve the case, anita and the guard are grateful and helpful, anita deputizes the PCs and gives them some authority and resources.