Martha Rendell killed 7-year-old Annie first. Her method was to put something in the child’s food that would result in a sore throat. It was alleged that she killed the children by swabbing hydrochloric acid on the backs of their throats, claiming it was medicine. This would inflame the throat until the child could no longer eat, and thus would starve to death. Annie died on 28 July 1907. Dr. Cuthbert issued a certificate stating the cause of death was diphtheria. After killing Annie, she turned her attention on Olive, aged 5. Olive died on 6 October 1907, and again Cuthbert issued a certificate stating the cause of death was diphtheria. In the winter of 1908 Rendell tried the same method on Arthur, the third son and youngest child still alive. Arthur, who was 14, took longer to succumb to the treatment, finally dying on 6 October 1908. Cuthbert asked permission for an autopsy. Rendell said she wanted to be present during the investigation. She stood by as the autopsy was performed, and the doctors found nothing to incriminate her. In April 1909, she turned her attention on the second son, George. It didn’t take long for the second son to complain of a sore throat after drinking a cup of tea. Rendell coated his tonsils with the syrup, frightening the boy, who ran to his mother’s place some streets away. Neighbours would enquire as to the boy’s whereabouts; however, his father Thomas Morris would state that he did not know.

Neighbours went to the police, and inspector Harry Mann conducted inquiries. Mann heard repeated references to the children’s having their throats painted, and Martha Rendell’s apparent indifference to their pain. One neighbour claimed he often peeked in the windows to see Rendell standing in front of the screaming victim, rocking back and forth as if in ecstasy. Mann located George Morris, who had claimed to have run away because his stepmother had killed his siblings and was trying to poison him with spirits of salts (i.e. hydrochloric acid). The inquiry was hampered by the period of time that had elapsed since the deaths, and because doctors could not say what effect swabbing with spirits of salts would have. Suspicions were further aroused when it was shown that Rendell had purchased large quantities of spirits of salts during the period of the children’s illnesses, but none since the last death. Armed with this information the detectives obtained permission to exhume the bodies and this was done on 3 July 1909. Police exhumed the bodies of the three children; diluted hydrochloric acid was found on the throat tissue.

Martha Rendell and Thomas Morris were both charged with murder, the former being sentenced to death by hanging. Rendell protested her innocence, maintaining that she was treating the children for diphtheria. Although Thomas Morris was also charged with the murders, he was acquitted; it was believed that, although he had purchased spirits of salts, he had not been aware of the crimes until after the children’s deaths. The jury wanted to find him guilty of being an accessory after the fact, but this was not allowed.