Andreas Lubitz
Andreas Lubitz and an Airbus cockpit Twitter/Getty Images

Lufthansa's CEO has described the air crash in which Germanwings co-pilot Andreas Lubitz deliberately flew an Airbus into French Alps on 24 March, killing 150 people, as "the most terrible event in the company's history."

Speaking at a press conference in Cologne, Germany, Spohr confirmed what investigators in Marseille had found: that the pilot had been prevented from re-entering the cockpit by co-pilot Andreas Lubitz, 28.

"The plane was crashed on purpose. The captain after reaching altitude left the cockpit and could not return. The co-pilot denied him access back into the cockpit to start the fatal descent into the French Alps," he said.

The 150 passengers on board may not have been aware of their impending doom, according to French prosecutor Brice Robin, as the plane descended gradually as if it were coming down to land before slamming into the mountains of Le Vernet, in the Provence Alps near Seyne-les-Alpes.

But as the terror of the situation suddenly became clear, the prosecutor said "we only hear screams in the last seconds. Death was instant."

Sandrine, the niece of Christian, a 59-year old victim from Belgium, said it was a relief that the passengers were unaware of the looming disaster until the final seconds, according to the black box voice recording.

"You can hear screams, but they didn't live through eight minutes of total horror," she told RTL radio, even if this was "a protocol not to panic people".

Spohr said: "It leaves us absolutely speechless. We are really deeply shocked and I wouldn't not have been able to imagine that the situation would have got even worse".

Describing cockpit safety measures introduced in commercial airliners in the wake of 9/11 attacks, Spohr said that doors were now reinforced, meaning they could not be broken down even by weapons. A code known to all the crew by heart allows access to the cockpit, however this can be overridden from within, he said.

He said that the pilot had managed to enter the code to gain re-entry, but had been prevented from entering. "You can never exclude such an individual event," said Spohr, adding "no system in the world could manage to do that".

Unlike in the US, he said, regulations in Europe did not stipulate that two people had to be in the cockpit at all times. Spohr also said that there had been no abnormalities in Lubitz's training and he had passed all psychological and physical tests.

However, he said that Lubitz had taken a number of weeks off during the initial stages of training six years ago. "During the training six years ago there was a long interruption then the person [Lubitz] was fit to continue and he went through the medical tests and examinations," said Spohr.

He insisted that training procedures at Lufthansa are sound, but they will nonetheless be re-evaluated.