re: Army Times - Bergdahl's attorney also fought for Army captain in espionage case

The 15-6 is usually used for more general inquiries- like what happened, the investigating officer may be looking for the specific facts, systematic issues, so in this case if a 15-6 officer is appointed he may be answering questions concerning how this can be stopped in the future, was there a failure in command.

The Article 32 investigating officer is looking at whether an individual should be brought up on charges- Should Berghdahl face Court Martial or something else.

Maybe the AR 15-6 is the same as the Navy Court of Inquiry, like what do you use if a ship runs aground, or a bunch of naval aviators have a massive blow out and offend some people.