SEYNE-LES-ALPES, France - Only the co-pilot of Germanwings Flight 9525 was in the cockpit when the Airbus A320 slammed into a steep mountainside in the French Alps, a French prosecutor confirmed Thursday, saying he had "intentionally" crashed the aircraft.

French prosecutor Brice Robin said in Marseille on Thursday that the German co-pilot, Andreas Lubitz, requested control of the aircraft about 20 minutes into the flight. The pilot then left the cockpit, leaving the co-pilot in full control of the plane.

Lubitz manually and "intentionally" set the plane on the descent that drove it into the mountainside in the southern French Alps. It was the co-pilot's "intention to destroy this plane," Robin said.

The Airbus A320, on a flight from Barcelona to Duesseldorf, began to descend from cruising altitude after losing radio contact with ground control and slammed into the remote mountain on Tuesday morning, killing all 150 people on board.

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In Washington, the State Department identified the third American who died in the crash as Robert Oliver. State Department spokesman Jeff Rathke told reporters that the department was reviewing its records to determine whether any other citizens were on the flight.

Asked if he considered the tragedy a suicide, Carsten Spohr, CEO of Germanwings' parent company Lufthansa, said Thursday that "if one person takes 149 people with him into death, it's another word, not suicide."

Robin confirmed a report by The New York Times that the pilot tried aggressively to get back into the cockpit, but was denied access.

The prosecutor said the pilot was heard pleading with the co-pilot over a telephone intercom system from the cabin for access, but that he got no answer from the co-pilot.

Eventually, according to The Times account, "you can hear he (pilot) is trying to smash the door down."

The flight voice recorder, one of the so-called "black boxes," from Germanwings Flight 9525, which crashed March 24 in the French Alps. BUREAU D'ENQUÊTES ET D'ANALYSES (BEA, FRANCE)

On the cockpit voice recorder, which reached French accident investigators on Wednesday morning, Robin said there is audio of the co-pilot breathing, and that breathing continues until the moment before impact.

"He (co-pilot) was alive until impact," said Robin.

In the German town of Montabaur, acquaintances said Lubitz was in his late twenties and showed no signs of depression when they saw him last fall as he renewed his glider pilot's license.

"He was happy he had the job with Germanwings and he was doing well," said a member of the glider club, Peter Ruecker, who watched him learn to fly. "He gave off a good feeling."

A file photo of the man believed to be Andreas Guenter Lubitz, who was the co-pilot of Germanwings Flight 9525, which crashed March 24, 2015, into the French Alps. AP

Lubitz had obtained his glider pilot's license as a teenager, and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Ruecker said. He described Lubitz as a "rather quiet" but friendly young man.

A U.S. law enforcement source told CBS News that Lubitz trained at an Arizona flight school from July until November of 2010, and was last in the U.S. in Oct. 2014 on a crew visa. He was licensed by the FAA to fly single-engine aircraft and gliders in the U.S., but it wasn't immediately clear whether he ever had.

As CBS News correspondent Mark Phillips reported, the new information changes completely the nature of the crash from a suspected accident, into something far more sinister, and will spark a criminal investigation.

In yet another sad twist for all involved, CBS Radio News correspondent Elaine Cobbe reported that families of the crash victims were brought near to the site of the crash Thursday to mourn. Lubitz' family also was there, but were being kept apart from the other families.

A video produced by Airbus, maker of the A320 passenger jet used on the Germanwings flight, shows how the cockpit security system was designed in 2002, after the Sept. 11 terror attacks. It shows that flight crew members in the cabin can access the cockpit with an code to open the door, but it doesn't deal with the possibility of what would happen if one of the pilots deliberately tries to lock out the other.

On the Airbus, like virtually every other commercial passenger jet since 9/11, the pilot or whoever has control of the cockpit has the ultimate override power to prevent others from entering from the plane's cabin.

Phillips noted that the information from the cockpit voice recorder contradicts directly what the French air accident investigators had said. They acknowledged there were voices and sounds on the recorder, but investigator Remi Jouty said they could not yet determine when those conversations took place in the course of the flight.

CBS News aviation consultant Chesley "Sully" Sullenberger, a former pilot and air crash investigator, said Thursday on "CBS This Morning" that in the U.S., there are protocols in place which require there to be at least two members of the flight crew in the cabin at all times. If the pilot or co-pilot needs to leave for any reason, a member of the cabin crew will step into the cockpit in the interim, for example.

In Europe, there is no such requirement.

Sullenberger said commercial pilots are some of the most heavily scrutinized professionals in the world, but in the end, it is "very difficult to predict who is going to act in a very bizarre and harmful way."

Sullenberger added that, given the nature of the panic at the cockpit door, the 144 passengers on the jet would have known what was going on.

"It would have been a terrifying number of minutes," he said.