Germanwings Flight 9525 co-pilot deliberately crashed plane, officials say

(CNN)The co-pilot of Germanwings Flight 9525 purposely crashed the plane into the French Alps on Tuesday, killing all 150 people on board, officials said Thursday.

"We at Lufthansa are speechless that this aircraft has been deliberately crashed by the co-pilot," said Carsten Spohr, CEO of Lufthansa, which owns Germanwings.

Marseille prosecutor Brice Robin said the co-pilot, 28-year-old German national Andreas Lubitz, apparently "wanted to destroy the aircraft."

It's unknown whether Lubitz planned his actions in advance, 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.

It's also unclear whether the pilot entered a code to try to get back into the cockpit, or whether Lubitz "put the lever on lock," which would have prevented the code from working, Spohr said.

What is known is that screaming could be heard on an audio recording for just the final few minutes of the flight.

Death was instantaneous for those on board when the plane plunged into the mountains, Robin said.

German authorities also say "that we have to assume" that Flight 9525 "was deliberately activated for a crash," German Chancellor Angela Merkel said Thursday. "This is hard to comprehend for most people. Nobody can imagine this."

The French government has asked the FBI to help investigate the crash, a law enforcement official said Thursday.

A search is underway for the plane's second "black box," the flight data recorder, which could shed more light on the plane's final minutes.

No clues about why co-pilot would crash plane

Investigators are baffled as to why Lubitz would have done this

Lufthansa does "not have any clues," Spohr said.

The picture of the plane's final minutes comes largely from what was discovered in the mangled cockpit voice recorder.

The pilot and co-pilot had normal exchanges during the flight, Robin said. When the pilot stepped out to go to the bathroom, he asked Lubitz to take over.

The most plausible explanation of what happened next is that Lubitz, "through deliberate abstention, refused to open the cabin door ... to the chief pilot, and used the button" to cause the plane to lose altitude, Robin said.

The disaster is not being described as a "terrorist attack," and the killing of 150 people would generally not be described as a "suicide" either, Robin said. Spohr agreed: "If a person kills himself and also 149 other people, another word should be used -- not suicide," he said.

Lubitz was not known to be on any terrorism list, and his religion was not immediately known, Robin said.

Lufthansa has no standard psychological testing after hiring

He had been with Germanwings since September 2013 and had completed 630 hours of flight time, the company said. Lubitz had trained at the Lufthansa flight center in Bremen, Germany.

He only had about 100 hours of experience on the type of aircraft he was flying, but he had all the necessary certifications and qualifications to pilot the aircraft alone, the prosecutor said.

He had passed medical tests, Spohr said. The audio recording showed his breathing to be steady, with no sign that he had a heart attack or other medical issue.

Lufthansa does not have standard psychological testing for pilots once they are hired, Spohr said. The company considers an applicant's psychological state when hiring, he said.

The co-pilot was "fully qualified to pilot the aircraft on his own," Robin added.

A man in Montabaur, Germany, who belonged to the same flight club as Lubitz, said he couldn't believe it. "The way I know Andreas, this is inconceivable," Peter Ruecker said Thursday.