To my knowledge, only operational factors or matters of national security could justify an exception.
Let's use the example of the helicopter crashes in Afghanistan. Our investigators on site may have been more careful in the drafting of their reports if they had good reasons to believe that the information could potentially be misused by our Afghan adversaries. Yet even in that specific case, the investigation reports produced were made public. That being said, the process is fairly long, as a whole year—sometimes even two or three years—may go by before an investigation is completed.