According to Bild on Thursday, investigators say the deadly fire began smouldering in the building’s cellar, and they have ruled out wiring defects as a possible cause.

Since the tragic deaths on February 3, suspicions that arsonists set the fire in a hate crime against the building’s Turkish residents have abounded. Turkey sent its own experts to help German officials with the investigation.

The paper writes that experts say the smouldering fire began under the second and third steps of the building’s cellar. The source of the fire remains unclear, but authorities have ruled out technical defects in the building as a possible cause. Arson is also thought to be unlikely, the paper writes.

Authorities expect the results of psychological analysis for two children who say they saw a man start the fire. Their testimonies would be the only concrete evidence of arson, Bild writes.