Only six American civilians were killed on the US mainland during WW2. They were Elsie Mitchell (who was pregnant at the time) and her Sunday school class of five children. They were killed by a Japanese fire balloon that landed near Gearheart Mountain in Oregon. The remnants of the balloon were discovered by the group while they were searching for a picnic location. The payload did not detonate upon landing, and investigators theorized that it was triggered when one of the victims kicked it.

Source: "Daddy's Gone to War": The Second World War in the Lives of America's Children by William Tuttle.