The Menace Within — Ursula Curtiss

When her aunt had been taken to hospital after a bad stroke, Amanda was left in charge of her house. Amanda knew about her aunt's dog but she didn't know about the secret underground shelter built beneath the house. This hiding place had been put to secret and illicit use by a workman who had known of its existence when it was first built, before Amanda's aunt bought the house. A murderer had been hidden in the secret room: a man who had recently killed a young woman : a man far from sane, who had a sane, loyal and dangerous ally in his half-brother. From these men's points of view it becomes imperative to eliminate Amanda, whose predicament has been made worse by her having undertaken to look after the baby of some friends who have gone on holiday. Amanda and the baby are hounded through empty, snowbound streets, cornered and threatened by these ruthless men.