Any truly thorough investigation is likely to expose lots of lawbreaking by the BND and German politicians, which is why any investigation is likely to go nowhere; or, at best degenerate into another "it's all the Amis fault" variety show for the distraction of the German masses.
Primary creator of the BND was FHO head Reinhardt Gehlen. He may have served the CIA in order to get his “Gehlen Org” to take off (full of former SD, SS and Gestapo officers, for the record), but when it became the BND under the West German government, they were cut loose in 1956 and allowed to become autonomous.