Here's Powell's summary of a book called "Claim of Privilege":

On October 6, 1948, a trio of civilian engineers joined a U.S. Air Force crew on a B-29 Superfortress, whose mission was to test secret navigational equipment. Shortly after takeoff the plane crashed, killing all three engineers and six others. In June 1949, the widows of the engineers filed suit against the government. What had happened to their men? they asked. Why had these civilians been aboard an Air Force plane in the first place?

But the Air Force, at the dawn of the Cold War, refused to hand over the accident reports and witness statements, claiming the documents contained classified information that would threaten national security. The case made its way up to the Supreme Court, which in 1953 sided with the Air Force in United States v. Reynolds. This landmark decision formally recognized the "state secrets" privilege, a legal precedent that has since been used to conceal conduct, withhold documents, block troublesome litigation, and, most recently, detain terror suspects without due-process protections.

Even with the case closed, the families of those who died in the crash never stopped wondering what had happened in that B-29. They finally had their answer a half century later: In 2000 they learned that the government was now making available the top-secret information the families had sought long ago, in vain. The documents, it turned out, contained no national security secrets but rather a shocking chronicle of negligence.

The story is also told in Act 2 of this week's This American Life.