The German co-pilot who is believed to have deliberately crashed a plane in the French Alps, killing 150 people, broke off his training six years ago due to depression and spent over a year in psychiatric treatment, a German newspaper reported on Friday.

The story in the German tabloid Bild came a day after French prosecutors said they believed Andreas Lubitz, a 27-year-old co-pilot at Lufthansa's budget airline Germanwings, had locked the captain out of the cockpit and steered the Airbus A320 airliner into its fatal descent.

Lufthansa Chief Executive Carsten Spohr acknowledged at a news conference on Thursday that Lubitz had broken off his training in 2009 but did not explain why. He said there was nothing in the pilot's background to suggest he was a risk. "After he was cleared again, he resumed training. He passed all the subsequent tests and checks with flying colors. His flying abilities were flawless," Spohr said.