It's unknown whether Lubitz planned his actions, Robin said. But he "took advantage" of a moment in which the pilot left the cockpit and "activated the descent," which can only be done deliberately.
CNN further reports that Transponder data shows that the autopilot was reprogrammed during the flight by someone inside the cockpit to change the plane's altitude from 38,000 feet to 100 feet, according to Flightradar24, a website that tracks aviation data.