PARIS — Andreas Lubitz was breathing, steady and calm, in the final moments of Germanwings Flight 9525. It was the only sound from within the cockpit that the voice recorder detected as Mr. Lubitz, the co-pilot, sent the plane into its descent.

The sounds coming from outside the cockpit door on Tuesday were something else altogether: knocking and pleading from the commanding pilot that he be let in, then violent pounding on the door and finally passengers’ screams moments before the plane, carrying 150 people, slammed into a mountainside in the French Alps.

Those clues led French prosecutors to say Thursday that the co-pilot had locked the pilot out of the cockpit and deliberately crashed the plane.

Image A photo taken from social media shows Andreas Lubitz, the co-pilot of the Germanwings jetliner that crashed in the French Alps on Tuesday.

The sound of Mr. Lubitz’s breathing indicated that he was conscious to the end, Brice Robin, the Marseille public prosecutor, said at a news conference. It appeared that Mr. Lubitz intended “to destroy the aircraft,” he said.

“The interpretation we can give at this time is that the co-pilot, through a deliberate act, refused to open the door of the cockpit to the commander, and activated the button that commands the loss of altitude,” Mr. Robin said.

Data from the plane’s transponder also suggested that the person at the controls had manually reset the autopilot to take the plane from 38,000 feet to 96 feet, the lowest possible setting, according to Flightradar24, a flight tracking service. The aircraft struck a mountainside at 6,000 feet.