The collapse happened as the city’s overseeing transport authority, the Kölner Verkehrs-Betrieben (KVB) and a large engineering and construction consortium (Arbeitsgemeinschaft, or Arge) including leading engineering firms Hochtief, Bauer, Keller, Brückner, Bilfinger, Wayss & Freytag and Ed. Züblin worked on tunneling and station construction on an ambitious north-south metro line expansion. As excavation was being carried out for the planned Waidmarkt station, the earth gave way underneath the six-story archive building above. A young baker’s apprentice sleeping after his morning shift and a 24-year-old design student were killed in nearby residential buildings caught in the collapse.

Cologne prosecutor Torsten Elschenbroich led off the district’s case by summarizing a 196-page indictment grounded upon on a lengthy official probe of the disaster, according to court reports from Spiegel and the German press agency. The probe points to a bent plate in a retaining wall meant to hold back soil and water during excavation. The retaining wall went up in 2005, according to the court reporting. The prosecution holds that a flaw in the wall is the culprit in what was basically a years-long slow-motion runup to the appearance of a sinkhole under the archive, during which a huge amount of gravel, mud and water—some 5,000 cubic meters, or 176,500 cu ft—moved through the excavation zone, taking the foundation from underneath the building above.

The defendants in the case include a former site foreman and leader for deep construction as well as an excavator on the project. German court rules hold that defendants are not formally named in public. Defense lawyers are pushing back against the theory about the retaining wall, asserting that contrary to both the probe and the indictment it has not been proven that the wall, flawed or otherwise, caused the archive collapse. A natural event such as a hydraulic groundbreaking caused by soil shifts could also be a cause, the Cologne judge heard from the defense.

The Arge is also disputing the prosecution’s findings. A spokesman for the consortium said it would not comment on the ongoing trial. A statement from Arge did say, however, that it is a complex question to determine how in just a few minutes so much earth could flow into the excavation space. It stated that the cause has been researched since 2009, is still unclear, and may yet take some years to determine. Until the cause is finally and definitively clear, the presumption of innocence should hold for all parties, Arge stated.

The archive collapse case is complex and controversial in Germany and, as with the original ambitious construction work on the Cologne metro, time pressure is at play now. As in France, Germany does not have a jury trial system—cases are rather heard by a judge or panel of judges and experts. The judge in this case, Michael Greve, has allotted 126 court days for the case, during which 93 witnesses and 10 experts are due to deliver testimony. On March 2, 2019, the legal statute of limitations expires on the archive collapse.