The Germanwings co-pilot, who is suspected of killing 149 people when he crashed a plane into the French Alps, rehearsed bringing down a previous flight.
Andreas Lubitz apparently practised descending an outbound flight from Dusseldorf to Barcelona on the same day he led hundreds to their death, a shock new report has claimed.
A source close to the French authorities investigating the 24 March tragedy told German newspaper Bild, flight 952, based on black box data, entered a "controlled descent that lasted for minutes and for which there was no aeronautical justification".
Investigators said the 27-year-old, who had previously been in therapy for suicidal tendencies, locked his captain out of the cockpit to deliberately crash the plane.
Full details of the interim report is due to be published by BEA, the French investigative crash agency.
Germanwings owner Lufthansa, declined to comment on the Bild report while the investigation continues.