A spokesperson for the federal authority in Karlsruhe told multiple German media on Thursday that the probe was being conducted on suspicion of "anticonstitutional sabotage."
Actions that cause a public transportation service to cease to function, either in whole or partially, are punishable with a fine or a prison term of up to five years under the German criminal code.
German rail operator Deutsche Bahn said on Saturday that a breakdown in the digital train radio system brought passenger and cargo trains in several states to a standstill. The company said later that severed cables pointed to an act of sabotage.