Chief prosecutor Wolfgang Giese says his office has opened a criminal investigation against the 39-year-old dispatcher.
Mr Giese said on Tuesday that according to investigators "had he (the dispatcher) behaved according to the rules, the trains would not have collided."
Mr Giese did not identify the dispatcher, who was interviewed by police on Monday in the presence of his lawyer, but he is being investigated on suspicion of negligent homicide, bodily harm and interference with rail traffic.
If convicted he could face up to five years in prison.
Mr Giese said i nvestigators believe the dispatcher, whose job involves directing rail traffic and ensuring safety on the tracks, sent a wrong signal to the trains. After noticing his mistake, he tried to alert the drivers using an emergency call but failed to prevent the crash, prosecutors said.
The two trains were supposed to pass each other at a station where the track was divided, but instead they crashed head-on on a curve early on February 9.
Mr Giese said a technical failure of the trains or signalling equipment had been ruled out, but investigators plan to reconstruct the accident to test their theory of what happened.
Fellow prosecutor Jurgen Branz said there was no indication the dispatcher was under the influence of alcohol or drugs at the time of the crash.
"What we have at the moment is a terrible error in this particular situation," he said.
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