Accidental triggering of a grounded balloon in Bly, Oregon caused the only known causalities of enemy action in the continental United States.
You can listen to the whole segment here http://www.radiolab.org/story/fu-go/
This is how RadioLab advertised the segment ...
Not how I would have put it, at all really. Forensic geologists talk about this case all the time. It's a classic. And the "silence" was pretty much self-imposed and temporary - it was not a national secret. The US government requested a news blackout on stories so that information about the landings didn't get back to 'the enemy.' (Though it turns out that several did get published - I was surprised by the number I found when I researched this story.)
The press blackout was officially ended with a single press release on May 22, 1945, when the Army and Navy disclosed information about the attacks “so that the public may be aware of the possible danger and to reassure the nation that these attacks are so scattered and aimless that they constitute no military threat” (Chicago Daily Tribune, 1945). The press release was widely reported in papers across the nation.
The news blackout turned out to be very important. If verification of any clear successes had reached Japan, there would have been motivation to continue the program - they reportedly had thousands more balloons ready for launch. As it was there were several important near misses.
The write-up that I did for my book is available here http://higheredbcs.wiley.com/legacy/college/bergslien/1405160543/notes/05ch-2.pdf
And Radiolab had also posted a nice gallery of images at http://www.radiolab.org/story/pictures-fu-go/
Just FYI here is the cover of my book.