Special Agent Oran Diemer was shot and killed in Hannibal, Missouri, while he and another railroad detective arrested a man for trespassing on a freight train.

They encountered the man in one of the train's boxcars while it was in transit. He requested to be allowed to purchase a ticket when they arrived at the next city and the detectives stated they would allow him to per the company's policy. When the train slowed as it entered Hannibal the man suddenly produced a handgun and shot Special Agent Diemer in the forehead, killing him. He then shot the second detective three times before jumping from the train near the Main Street crossing. Despite his wounds, the wounded detective jumped from the train and returned fire at the subject as he fled.

Within a few hours the suspect, 35, who was armed with a .38 caliber revolver with four empty chambers, was apprehended by the city marshal in Monroe City, and positively identified by the wounded detective. On January 28, 1918, he was convicted of first degree murder and sentenced to life. He was paroled on January 23, 1953.

Special Agent Diemer had served as a special agent for the railroad for two years. He was survived by two brothers. Interred: Greenwood Cemetery, Palmyra, Marion County, MO.