We now know a little bit more about the Germanwings pilot who deliberately crashed into a mountain, killing himself and 149 other people; but friends and family are still baffled as to why 28 year old Andreas Günter Lubitz would lock himself into the cockpit and initiate the plane’s deadly descent.

Lubitz reportedly had taken a hiatus from his pilot training in 2008 due to “burnout.” Friends close to Lubitz told investigators that the break came about as a result of Lubitz’s “depression.” That break lasted for several months, but he was deemed “100% fit to fly” well before this week’s tragedy.

Lufthansa CEO Carsten Spohr went into detail about the hiatus and the security precautions the airline takes with all its employees:

“We are shaken. This is our worst nightmare, that such a tragedy could happen in our group,” CEO Carsten Spohr told reporters in Cologne.

The company carefully vetted its crews and did not only test them on technical abilities but also made psychological checks.

He said the co-pilot had taken an 11-month break, during which he worked as a flight attendant but that he passed all the relevant checks upon restarting. Spohr said the break was not unusual and declined to give details of the reasons behind the pause in training.

“No matter your safety regulations, no matter how high you set the bar, and we have incredibly high standards, there is no way to rule out such an event,” Spohr said. “This is an awful one-off event.”

Officials are slowly ruling out the possibility that this was an act of terrorism—Lubitz had no known affiliations with any terror organizations—but the evidence that has been reviewed so far is no less horrifying for it.

The Daily Mail has video of French prosecutor Brice Robin describing how the co-pilot came to be in control of the plane:

Salvage teams have not been able to locate the plane’s data recorder, but prosecutors have listened to and analyzed the contents of the cockpit voice recorder. The reports of what happened in the moments leading up to the crash are terrifying:

“This was voluntary, this was deliberate,” Marseille prosecutor Brice Robin said. “He refused to open the cabin door in order to let the pilot back in. I repeat. He refused to let the pilot back in. He is the one who pressed the button that allowed the plane to begin descending and lose altitude.”

The information was obtained from the cockpit voice recorder of doomed Flight 9525, which suddenly began an eight-minute descent before smashing into the mountains Tuesday. The data recorder for the Barcelona to Düsseldorf, Germany, flight has not yet been found.

Robin stressed the actions were deliberate. He said passengers could be heard screaming in fear.

“We start hearing banging, someone actually trying to break the door down,” Robin said. “That’s why the alarms were let off — because these were protocols that were put in place in case of any terror attack.”

We’ll keep you updated as new developments come available.