Keepers of the Cave by Gerri Hill

While the investigations go on in Dallas and Baton Rouge after the disappearance of a senator's daughter, FBI agents CJ Johnston and Paige Riley are assigned to the sleepy backwoods of East Texas. Random disappearances dating back fifty years and more raise red flags that point to the tiny, isolated community of Hoganville, where CJ and Paige go undercover at an all-girls school. Feeling the assignment is nothing more than a dead-end, they integrate themselves into the lives of the teachers and staff, more concerned with getting past the one-night stand they shared six months earlier than actually thinking Hoganville was somehow connected to the disappearances. But soon, the odd behavior of the townspeople has them convinced something sinister lurks there. Something, perhaps, that even the residents of Hoganville don't know about.