Troubling Warning Signs From Germanwings Co-Pilot Who Crashed Plane Surface

'Depressed' and 'burned out' are the troubling words describing the co-pilot who deliberately flew the jetliner into the side of a mountain.

His name is Andreas Lubitz and he is 28 years old.

The revelation that Lubitz intentionally crashed the jet is horrifying the world.

He planned to get married next year, and he trained at a flight school in Pheonix, Arizona.

Lubitz worked for Germanwings, a subsidiary of Lufthansa, for only 18 months.

He lived with his parents in a house in Germany. The family name is on the front door.

Authoritities released bone-chilling details of the flight's final minutes revealed by the cockpit voice recorder.

When the plane's captain left the cockpit to go to the bathroom, his co-pilot locked him out. The captain can be heard knocking on the door to get back in, but gets no answer. He started pounding on the door, but there was only dead silence. It went on like this for several agonizing minutes, and the passengers can actually be heard screaming as they realize the plane is going down.

Cockpit doors are locked from the inside and have been ever since the 9/11 terrorist attacks.

The door can usually be opened from outside using a secret code, but former Delta Airlines captain Eric Schoneberger said the pilot can change that.

He told INSIDE EDITION, "The armored doors have a manual override lock inside the flight deck that precludes opening the door from the cabin. If they believe they are under attack that would be the last form of defense."

The cockpit door is basically inpenetrable.

John Hansman, a Professor of Aeronautics at MIT said, "You would not be able to get through that door. Most of them are steal-plated. Some of them are bullet-proof."

Three Americans were killed in the Germanwings crash, 23-year-old Emily Selke of Virginia and her mother Yvonne both died. So did Robert Calvo, a father of two.

Two prominent opera singers from Europe were also among the victims.

One-hundred-and-fifty people all dead because of the deranged actions of a lunatic.