The co-pilot of the doomed Germanwings jet barricaded himself in the cockpit and "intentionally" sent the plane full speed into a mountain in the French Alps, ignoring the pilot's frantic pounding on the door and the screams of terror from passengers, a prosecutor said Thursday.

Co-pilot Andreas Lubitz's "intention (was) to destroy this plane," Marseille prosecutor Brice Robin said, laying out the horrifying conclusions reached by French aviation investigators after listening to the last minutes of Tuesday's Flight 9525.

The Airbus A320 was flying from Barcelona to Duesseldorf when it began to descend from cruising altitude of 38,000 feet after losing radio contact with air traffic controllers. All 150 on board died when the plane slammed into the mountain.

Robin said the pilot, who has not been identified, left the cockpit, presumably to go to the lavatory, and then was unable to regain access. In the meantime, Lubitz, a 28-year-old German, manually set the plane on the descent that drove it into the mountain.

Robin said the commander of the plane knocked several times "without response." He said the door could only be blocked manually.

"The most plausible, the most probably, is that the co-pilot voluntarily refused to open the door of the cockpit for the captain and pressed the button for the descent," Robin said.

He said the co-pilot's responses, initially courteous in the first part of the trip, became "curt" when the captain began the mid-flight briefing on the planned landing.

The information was pulled from the black box cockpit voice recorder, but Robin said the co-pilot said nothing from the moment the commanding pilot left.

"It was absolute silence in the cockpit," he said.

During the final minutes of the flight's descent, pounding could be heard on the cockpit door as plane alarms sounded but the co-pilot's breathing was normal the whole time, Robin said.

"It's obvious this co-pilot took advantage of the commander's absence. Could he have known he would leave? It is too early to say," he said.

He said Lubitz had never been flagged as a terrorist and would not give details on his religion or ethnic background. German authorities were taking charge of the investigation into the co-pilot.

Robin said just before the plane hit the mountain, the sounds of passengers screaming could be heard on the audio.

"I think the victims realized just at the last moment," he said.

The A320 is designed with safeguards to allow emergency entry if a pilot inside is unresponsive, but the override code known to the crew does not go into effect - and indeed goes into a lockdown - if the person inside the cockpit specifically denies entry, according to an Airbus training video and a pilot who has six years of experience with the jets.

Cockpit requirements differ in Europe

Airlines in Europe are not required to have two people in the cockpit at all times, unlike the standard U.S. operating procedure after the 9/11 attacks changed to require a flight attendant to take the spot of a briefly departing pilot.

In the German town of Montabaur, acquaintances told The Associated Press that Lubitz appeared normal and happy 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 Lubitz learn to fly. "He gave off a good feeling."

News of how investigators thought the plane crashed shocked the families, the airlines and everyone who heard the chilling, blow-by-blow description from the prosecutor. Lufthansa CEO Carsten Spohr said the airline was already "appalled" by what happened.

"I could not have imagined that becoming even worse," Spohr said in Cologne. "We choose our cockpit staff very, very carefully."

The families of victims were briefed about the conclusions just ahead of the announcement.

"The victims deserve explanations from the prosecutor," Robin said. "(But) they have having a hard time believing it."

Robin said the second black box still had not been found but remains of victims and DNA identification have begun, he said.

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.

Lubitz' recently deleted Facebook page appeared to show a smiling man in a dark brown jacket posing in front of the Golden Gate Bridge in California. The page was wiped sometime in the past two days.

Lufthansa said Lubitz joined Germanwings in September 2013, directly out of flight school, and had flown 630 hours. The captain had more than 6,000 hours of flying time and been a Germanwings pilot since May 2014, having previously flown for Lufthansa and Condor.

The circumstances of the crash are likely to raise questions anew about the possibility of suicidal pilots.

Three Americans killed in crash

Three Americans were killed in the crash, including a U.S. government contractor and her daughter, the State Department said Wednesday.

Identified victims were Yvonne Selke of Nokesville, Virginia, an employee for 23 years at Booz Allen Hamilton Inc. in Washington, and her daughter, Emily Selke, a recent graduate of Drexel University in Philadelphia. The U.S. government did not identify the third American it said was on the plane.

Yvonne Selke performed work under contract with the National Geospatial-Intelligence Agency, the Pentagon's satellite mapping office, Booz Allen and the Defense Department confirmed in statements after the AP had reported her identity and employment.

"Every death is a tragedy, but seldom does a death affect us all so directly and unexpectedly," NGA Director Robert Cardillo said. "All of us offer our deepest condolences and will keep her family and her colleagues in our thoughts."

Booz Allen's chief personnel officer, Betty Thompson, described Selke as "a wonderful co-worker and a dedicated employee who spent her career with the firm."

Friends and co-workers of Selke's circulated a photograph of her showing a smiling, middle-aged woman with brown hair and eyeglasses, and a photo of Emily showing a blond young woman with dark eyes and a bright smile. They described Selke as a diligent and generous worker who regularly brought cookies to co-workers.

A person who answered the phone at Selke's home said the family was not providing any information.

Emily Selke was a "go-getter" who was interested in festival management while in school, said Xela Batchelder, a Drexel professor who taught her. Batchelder said Emily Selke participated in Fringe University, which holds courses around the Edinburgh Fringe Festival, an arts festival in Scotland. She also helped put together the Pittsburgh Fringe Festival.

"She just kept being a self-learner even after school," Batchelder said. "We're just really upset, and we'll really miss her and she's just an amazing person."

Alex Tyler, who was a friend at Drexel, sent a statement to the AP that she said was written by a group of her close friends.

"Emily was a truly amazing woman, and we were honored to have her in our lives," the statement said. "She was an incredibly fun person who never shied away from a challenge. Emily grew our hearts with acceptance of everyone and grew our minds with new music and her love of travel. She was a light that shined in our lives, and we will always remember her with a smile on her face and a funny story to share."

Drexel University said in a statement that Emily Selke graduated with honors in 2013 and was a music industry major. Her sorority at Drexel, Gamma Sigma Sigma, said in a statement on its Facebook page with a photo of Emily that it was mourning her loss and said she "always put others before herself and cared deeply for all those in her life."

State Department spokeswoman Jen Psaki said the U.S. was reviewing records to determine whether any other U.S. citizens might have been on board the flight.

"We extend our deepest condolences to the families and loved ones of the 150 people on board," Psaki said.

Further details about Yvonne Selke's work for the secretive Pentagon agency were not immediately available. Most information about her assignment and contact information had already been removed Wednesday from Booz Allen's internal network.

Minute by minute, the demise of doomed German flight

Here is a look at the short journey and violent end of the German budget airline flight that crashed into the French Alps with 150 people aboard, based on information from French accident investigators, government officials and the airline Germanwings, a subsidiary of Lufthansa:

Times listed are Central Europe Time

Monday, March 23

The plane, a Germanwings Airbus A320, undergoes a routine safety check at Duesseldorf Airport.

Tuesday March 24

8:57 a.m.

The 24-year-old A320 single-aisle jet arrives in Barcelona from Duesseldorf.

10:01 a.m.

Germanwings Flight 9525 takes off from Barcelona, slightly delayed because of airport congestion, for the return trip to Duesseldorf, about 1,300 kilometers (800 miles) away.

Aboard were 144 passengers, two pilots and four cabin staff. Among them: babies, a group of German high-school students, an American contractor and her daughter, three generations of the same family.

10:30 a.m.

The pilots make their last contact with air traffic control in a routine message seeking and obtaining permission to continue course. At the time, the aircraft had reached its cruising altitude of 38,000 feet, and was over the Mediterranean Sea approaching the region around Marseille.

10:31 a.m.

The plane begins its unexplained descent above the French coast, just as its flight path crosses over land.

10:35 a.m.

Air traffic controllers try to contact the pilots, but receive no response.

10:40 a.m.

The last radar position of the plane is registered, at 6,175 feet - only slightly higher than the peaks of the southern French Alps below. The French Air Force sends a Mirage fighter jet to the plane's last known location. An air force official says the Mirage arrived too late to help, and never crossed paths with the A320.

About 12 noon

Rescuers in helicopters spot the debris, scattered across a ravine and the slopes of the Col de Mariaud. Amid the pulverized remains, there were no survivors.