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, 28, 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.

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.

What we know about Germanwings flight 4U9525's co-pilot

Twenty-eight-year-old Andreas Lubitz had been flying with the Germanwings airline for just 18 months when, according to officials, he intentionally brought his Airbus A320 crashing into the French Alps.

As officials struggle to make sense of the tragedy, a picture of the co-pilot allegedly at the controls of Flight 4U9525 has begun to emerge.

The man who is believed to have be deliberately caused the deaths of 150 people lived in the German town of Montabaur, just 90 minutes drive from Dusseldorf Airport where his doomed plane was due to land.

Lubitz belonged to the local LSC Westerwald flying club in Montabaur, and the group said on its website it had learned "with horror" that it had lost a "long-time member" to the Alps tragedy.

As a teenager, Lubitz obtained his glider pilot's license and he was later accepted as a pilot trainee with Lufthansa, one acquaintance from Montabeur told The Associated Press. Peter Ruecker said he had watched Lubitz learning how to fly, describing the young pilot as "rather quiet" but friendly.

"He gave off a good feeling," Ruecker told the news agency.

Lubitz started his training with the organization in 2008 and at one point worked as a flight attendant, the airline's Chief Executive Carsten Spohr confirmed Thursday.

While Lubitz had interrupted his training at some point for several months, the airline would not specify why, Spohr said that was not unusual and that the young German was re-evaluated before his training resumed. He passed all his tests.

After coming out of flight school with Lufthansa, Lubitz joined Germanwings in September 2013. Since then, he had accrued some 630 flight hours, according to the airline.

Spohr stressed that his company chose staff "very carefully" and said there was no clear indication of what might have caused Lubitz to bring down the plane.

Lubitz also possessed a single-engine private pilots license by the FAA on Jan. 6, 2012, according to the agency. One of the requirements for that qualification was English proficiency and a German pilots license.

An Andreas Guenter Lubitz was included in the "prestigious" FAA Airmen Certification Database in September 2013, according to the Aviation Business Gazette. The database recognizes airmen "who have met or exceeded the high educational, licensing and medical standards established by the FAA," the Gazette said.

While the details in the Gazette matched those of the first officer on board the crashed Germanwings plane, NBC News was not immediately able to verify it was the same man. (Source: NBC News)

Story Date: March 27, 2015