Düsseldorf prosecutor Christoph Kumpa said on Monday authorities were closing the case after determining that no one other than Lubitz could have caused the intentional crash. They had been looking into whether somebody still alive might have been culpable.

"The investigation has found no reason to prosecute any living person," Kumpa told German news agency DPA.

Co-pilot Lubitz, who was 27, locked the pilot out of the cockpit and then brought down Germanwings Flight 9525 in the French Alps on March 24, 2015, killing all 150 passengers and crew members. It was later revealed that he had been suffering from suicidal tendencies and had sought professional help several times.

Lawsuit in the US

Lawyers representing families of some of the victims say Lubitz should have been more closely monitored and should not have been allowed to fly. The revelations also led to calls in Europe for new laws on screening and monitoring of pilots.

However, prosecutors said they had found no cause for fault on the part of the doctors, Germanwings, its parent company Lufthansa, or the German aviation authorities.

In April, relatives of the people killed filed a lawsuit against the US flight school that provided training to Lubitz, accusing the Airline Training Center of Arizona (ATCA) of negligence and "failing to apply to its own well-advertised 'stringent' standards to discover the history of Lubitz's severe mental illness."

French authorities have been conducting a separate investigation of the crash.

blc/msh (dpa, Reuters, AP)