Which real life event inspired the back story to Agatha Christie's "Murder on the Orient Express"?

In investigating the murder on the train Hercule Poirot ascertains the victim's real identity, and possible motives for his murder. A few years before, in the United States, three-year-old heiress Daisy Armstrong was kidnapped by a man named Cassetti. Cassetti eventually killed the child, despite collecting the ransom from the wealthy Armstrong family. The shock devastated the family, leading to a number of deaths and suicides. Cassetti was caught, but fled the country after he was acquitted. It's suspected that Cassetti used his considerable resources to rig the trial. Poirot concludes that the victim was, in fact, Cassetti.

The Armstrong kidnapping case was based on the actual kidnapping and murder of Charles Lindbergh's son in 1932, just before the book was written. An innocent, but perhaps loose-lipped, maid employed by Mrs. Lindbergh's parents was suspected of involvement in the crime. After being harshly interrogated by police, she committed suicide.