Peter Nielsen was the only air traffic controller on duty at the time of the accident; he realised the danger but too late.

Some of the defendants, who have been not identified because of Swiss privacy laws, blamed Nielsen for the crash, accusing him of not following proper procedures.

"If two air traffic controllers had been present, the accident could have been prevented," the presiding judge said on Tuesday.

A second controller had been on an authorised break at the time of the accident, he was one of the four men acquitted.

'Culture of negligence'

Prosecutors said a culture of negligence and lack of risk awareness at the company contributed to the accident.

"The Germans had left everything to the Swiss and the Swiss had left everything to the Germans, the controller had no licence, the sector wasn't regulated and no one was doing safety analysis because they had left it up to the other side," David Gleave, an air crash investigator, told Al Jazeera.

A Russian passenger jet and a cargo plane
collided over southern Germany [File: EPA]
"Only when we start putting people in jail will safety be taken seriously."

Nielsen told investigators that he had been working under stressful conditions on the night of the crash.

A colleague was on a break and maintenance on the air traffic control system had affected monitoring and communications, he said.

Prosecutors also said that neighbouring control centres were not informed that the main telephone connection to Skyguide was out of order on the night of the incident.

German officials tried to warn Nielsen, but could not reach him.

But Nielsen was not among those on trial. In 2004, he was stabbed to death by a Russian man whose wife and children had died in the crash.

Vitaly Kaloyev is currently serving a prison sentence of five and a quarter years for the killing.