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The co-pilot of the crashed Germanwings plane appears to have "intentionally" brought the plane down while his captain was locked out of the cockpit and banging to be let back in, prosecutors said Thursday.

German Chancellor Angela Merkel said the revelations added a "new, simply incomprehensible dimension" to the tragedy, adding that "something like this goes beyond anything we can imagine."

First Officer Andreas Lubitz, 27, was alone at the controls of the Airbus A320 as it began its rapid descent, Marseille Prosecutor Brice Robin told a news conference.

Passengers' cries were heard on the plane's cockpit voice recorder in the moments just before the plane slammed into the French Alps, Brice said.

"Banging" sounds also were audible, he said, suggesting the captain was trying to force his way back into the cockpit. However, the reinforced cockpit door was locked from the inside and could not be overridden, even with a coded entry panel.

“If he had been able to open this door, the captain would have done it," Brice said.

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Lubitz, a German national from the town of Montabaur, "didn’t say a word" during the descent, according to Brice, who said no distress signal or radio call was made.

“There was no reason to put the plane into a descent, nor to not respond to… air traffic controllers,” he said. "Was it suicide? I’m not using the word, I don’t know. Given the information I have at this time … I can tell you that he deliberately made possible the loss of altitude of the aircraft."

Three Americans were among the 150 people killed in the disaster, whose victims included sixteen high school children on an exchange visit, babies, businessmen and two opera singers.

The current interpretation, Brice added, is that the co-pilot had "a desire to destroy this plane” though there was nothing to indicate a terrorist connection.

Germanwings' parent company, Lufthansa, earlier said Lubitz joined the airline in September 2013, directly after training, and had flown 630 hours.

His captain was an "experienced" pilot, with more than 10 years' experience with the organization and more than 6,000 flight hours on the Airbus model.

Many airlines, especially U.S. carriers, have a flight attendant sit in the vacant seat if a pilot leaves, for example during a bathroom break.

While Lufthansa earlier Thursday would not comment on its cockpit security procedures, it said it followed rules set out by German authorities that allow temporary absence from the flight deck.

Former pilots and aviation experts told NBC News that most planes have coded entry door controls, but these can be overridden with a double lock — a practice implemented industry-wide after the 9/11 attacks.

“The cockpit has the ultimate control of the door,” said former pilot Captain John Cox. “If it is placed in the override mode then no matter what is done with the code pad, the door will remain locked. The security people were very firm on the need for the flight deck to remain the ultimate authority.”

“It’s likely that an airline like Lufthansa will have fitted the highest specification of security technology,” said David Gleave, an aviation safety investigator based at Loughborough University near Leicester, England.

“These reinforced doors are designed to be very strong — they can’t be smashed open. That’s the point of them.”

Robin said that families of the victims had been informed of the investigations' developments. His remarks to the press came as relatives of the 150 victims of Tuesday’s crash were aboard special flights bringing them to the crash site.