Germanwings Flight 4U 9525 was “deliberately crashed” by the aircraft’s first officer, the Marseille-based prosecutor leading the criminal probe into the March 24 occurrence in the French Alps says.

“I have a problem calling this a suicide when you kill another 150 people,” Brice Robin said at a March 26 briefing. Robin, who has has full access to the audio file found on the cockpit voice recorder recovered only hours after the crash, revealed evidence that he believes proves the first officer was alone on the flight deck, alive, and likely not incapacitated during the flight’s fatal descent.

Robin bases his conclusion on several factors, led by the first officer not allowing the captain back into the cockpit followed by a controlled descent that seems consistent with a deliberate act.

The aircraft took off as Flight 4U 9525 from Barcelona at 10:01 a.m. local time with a total of 150 people on board. The aircraft reached its cruising altitude of 38,000 ft. at 10:27 a.m. Its last ATC contact took place at 10:30 a.m. and was a routine communication which cleared the aircraft for the IRMAR waypoint around 25 nm south of Barcelonnette, France. One minute later, the aircraft started the unexpected descent. The aircraft lost around 30,000 ft. in altitude in 8 min.—a relatively steep descent, but well within the aircraft’s normal flight envelope. Its last recorded altitude was 6,175 ft., slightly higher than where it hit the side of a mountain.

The first officer, Andreas Lubitz, was 28 years old and had only 630 airline flight hours. He was trained by Lufthansa and started flying as a first officer on the A320 family in September 2013. According to company sources, he had to interrupt his pilot training for a period of time for “medical reasons, ” that were not specified. He had no issues during his time as a line pilot, the sources said.

Robin confirmed that the two pilots conducted a normal and friendly conversation before the captain left the cockpit to use the lavatory. The first officer did not speak after the captain left.  Knocking and banging on the cockpit door—presumably from the captain seeking to re-gain access to the flight deck—is audible on the cockpit voice recorder. The noise also leads the prosecutor to believe that the captain and others tried unsuccessfully to smash the door with force.

Robin said the recording picked up normal breathing by the first officer and screams from the cabin in the flight’s final moments. Robin believes they started screaming when they realized what was happening. He effectively ruled out terrorism. Criminal probes of fatal aircraft accidents are commonplace in France, and Robin’s involvement in the probe started before any evidence of a crime came to light.

The Airbus A320 cockpit door can be opened from outside using an emergency code entered into a keypad. At Lufthansa and Germanwings, pilots and cabin crew are aware of the code. Once entered, the door can be opened from outside after about 30 seconds, but this can be overridden from inside the cockpit by moving the door button to the locked position. The first officer could have used this device to ensure his plan was carried out.

If Robin’s theory is confirmed as the investigation progresses, the industry could be facing a debate whether the introduction of secure cockpit doors after the September 11 terrorist attacks in the U.S. has created a new safety issue.

There have been four crashes since 1994 that investigators believe can most likely be explained by deliberate pilot action. Affected were a Royal Air Maroc ATR-42 in 1994, a Silk Air Boeing 737 in 1997, an Egyptair Boeing 767 in 1999 and most recently a Linhas Aéreas de Mocambique (LAM) Embraer 190. In that latest November 2013 event, the co-pilot left the cockpit to use the lavatory and the captain manually changed the altitude preselector from 38,000 ft to as little as 592 ft. and moved the throttle into the idle position.

Deliberate action by pilots is also still pursued as one of the theories explaining the disappearance of Malaysia Airlines Flight MH370 last year.