U.S. Kunduz, Afghanistan Hospital bombing investigation

In a hearing this week Congress asked for the cockpit tapes of the Kunduz bombing. DoD investigators have them and listened to them but refused to provide them to Congress citing an ongoing investigation.
Is the DoD an independent branch of government that does not have to answer to Congress?
http://www.thedailybeast.com/articles/2015/10/10/afghan-hospital-attack-s-secret-tapes.html