They determined that the construction tunnel was miraculously missing (as in, according to the paper trail, they were supposed to be there but weren't) a vast number of the pumps that were meant to keep the gravel from being washed away by the ground water (leading to the gravel being washed away, hollowing out the ground and, well, gravity took its course).
Thanks to bureaucracy and politics, it's still not clear who is officially to blame, although it's mind-numbingly obvious that it's a mix of really stupid decisions in the planning stages ("let's run the subway line right underneath an extremely important building through an area that almost entirely consists of gravel and water") and corruption at some level of the building companies (which seems to be SOP for public building projects in Germany).
Let's just say that most parties involved are pretty happy the investigation hasn't really been going anywhere over the past five years, which is entirely coincidentally enough time to allow all politicians that might be blamed to transition to greener pastures (communal elections in the Land are held every five years).