Arnold was a Luddite, protesting against the World Exhibition and the inventions it would display. He had short gray hair, green eyes, muttonchops, and a mustache. At the time of his death, he donned a brown suit with a white-collared shirt, a green neckerchief and a Luddite pin.
Arnold was found dead inside the Inventors' Hall with two burn marks on his chest. Per Dick, Arnold died of cardiac arrest when his heart, weakened by age, received an electrical shock. By looking at the burn marks, Dick concluded that the killer fired the electric charge weapon with their right hand, meaning the killer was right-handed.
Next to the body, Maddie and the player found the electric charge weapon. As Charlie confirmed it matched the wounds on Arnold's body, it was filed as the murder weapon. It had a greasy substance that (per Viola) was a lubricant for bone-shakers, meaning the killer rode a bone-shaker.
Relationship with suspects
Per aristocrat Leopold Rochester, Arnold was a known Luddite. Leopold had invited Arnold to the World Exhibition because he wanted to educate him on the marvels of science, but had been stood up. Inventor Stanley Spark had been accused by Arnold of mistreating his workers. Inventor Celine Georges had been ambushed by Arnold's gang of Luddites and had created a trap for them out of retaliation.
Arnold's daughter, waitress Susie Nottingham, grieved for her father's murder and was discovered to have met a gentleman lover at the Inventors' Hall, where Arnold had been killed. Susie's boyfriend was revealed to be the Squad's inventor, Charles Dupont.
Killer and motives
The killer turned out to be Susie.
After denying involvement, Susie admitted to the crime and said that Arnold was not the saboteur. As a Luddite, Arnold refused to let her daughter become an inventor and doubted her intellect. When he saw Susie leaving the Inventors' Hall, Arnold confronted her and they argued. In a fit of anger, Susie unknowingly killed her father with an electric charge weapon she had invented. Judge Takakura sentenced her to 7 years in jail.
- Arnold's death at the hands of his daughter is one of the instances of domestic homicide in Mysteries of the Past.