The Prisoner in the Castle by Susan Elia MacNeal

Maggie Hope has been banished to Killoch Castle, which is on an island off the west coast of Scotland. She and ten others have been imprisoned on the island because of wartime secrets they know or operations they have been part of. The British government has isolated them so no one knows where they are, even their families and friends, but someone staying at the castle is picking off his or her fellow inhabitants one by one. Why is this happening and will the person be stopped before they are the only one left standing? The Prisoner in the Castle is the 8th book in the Maggie Hope mystery series.