On 6 June 1949, the body of two-year-old Clive Mangnoson was found in a sack in the Largs Bay sand hills, about 20 kilometres (12 mi) up the coast from Somerton. Lying next to him was his unconscious father, Keith Waldemar Mangnoson. The father was taken to a hospital in a very weak condition, suffering from exposure; following a medical examination, he was transferred to a mental hospital.
The Mangnosons had been missing for four days. The police believed that Clive had been dead for twenty-four hours when his body was found. The two were found by Neil McRae of Largs Bay, who claimed he had seen the location of the two in a dream the night before.
The coroner could not determine the young Mangnoson's cause of death, although it was not believed to be natural causes. The contents of the boy's stomach were sent to a government analyst for further examination.
The boy's mother, Roma Mangnoson, reported having been threatened by a masked man, who, while driving a battered cream car, almost ran her down outside her home in Cheapside Street, Largs North. Mrs Mangnoson stated that "the car stopped and a man with a khaki handkerchief over his face told her to 'keep away from the police or else.'" Additionally a similar looking man had been recently seen lurking around the house. Mrs. Mangnoson believed that this situation was related to her husband's attempt to identify the Somerton Man, believing him to be Carl Thompsen, who had worked with him in Renmark in 1939.
J. M. Gower, secretary of the Largs North Progress Association received anonymous phone calls threatening that Mrs. Mangnoson would meet with an accident if he interfered while A. H. Curtis, the acting mayor of Port Adelaide received three anonymous phone calls threatening "an accident" if he "stuck his nose into the Mangnoson affair." Police suspect the calls may be a hoax and the caller may be the same person who also terrorised a woman in a nearby suburb who had recently lost her husband in tragic circumstances.
Soon after being interviewed by police over her harassment, Mrs. Mangnoson collapsed and required medical treatment.