The Fortkeeper's Keys — Angeli Perrow

In this 6th book in the Key Mystery Series, Nick, Mandy and Peyton are caught in a web of intrigue at Old Fort Knox in Maine. They witness a mysterious figure roaming the dark passageways of the fort. Is it a real person determined to wreck the first Summer Fright Night? Or is it the ghost of a soldier who was caretaker of the fort in the 1800s? A strange symbol written on objects in the fort, leads them to investigate rumors of witchcraft across the river in Bucksport. The house where they are staying is right next to an old cemetery containing the gravestone of the town's founder, supposedly cursed by a witch. Trying to solve two mysteries -- one in the past and one in the present -- they search for a secret tunnel, the identity of the fort's intruder, and what the ancient keys they find unlock. Will they discover the answers before it is too late?