PARIS—Andreas Lubitz was alone in the cockpit, breathing in silence, as his captain pounded on a locked door and passengers screamed. Those chilling sounds—captured in a black-box recording—have left French investigators with little doubt that the crash that killed 150 people aboard Flight 9525 was deliberate.

Lead prosecutor Brice Robin said on Thursday he suspected Mr. Lubitz, the 27-year-old Germanwings co-pilot, locked the captain out of the cockpit, programmed the A320’s descent and slammed it into an alpine ridge at...