Jean-Sébastien Beaud didn't know what he'd find when he descended, by a rope dangling from a helicopter, onto a steep mountainside in the French Alps. Twenty minutes earlier, Beaud and three colleagues from the mountain rescue squad of the gendarmerie had received a call from an air-traffic control center in Lyon, telling them that a plane had disappeared from the radar screen over the Massif des Trois-Évêchés, a range of 9,000-foot peaks northwest of Nice. Now, at 11:10 A.M. on Tuesday, March 24, 2015, Beaud was lowered from the four-seat chopper and set down gently on the rock face. Plumes of smoke and small flames rose from debris scattered across the slope, and the odor of jet fuel thickened the air around him. A tall and athletic man in his early thirties, with a faint mustache and goatee, Beaud moved cautiously down the field of black scree, making a mental inventory of what he saw: a human torso, shoes, suitcases, seats, bits of fuselage, and everywhere, detached hands and feet. He could tell immediately that an aircraft had smashed full speed into the mountain and been obliterated. Rattled but focused on the task at hand, he clicked on his radio and notified headquarters: There could not possibly be any survivors.

Moments later, Beaud came across the plane's registration plate, which he noted was German. He crossed the slope and worked his way up a gully to the likely point of impact. He was under orders not to touch any evidence, so each time he encountered a fragment of a human being—including, horrifically, a few scattered faces that had peeled off skulls like masks—he planted a small colored peg in the ground. Twenty-five minutes after he landed on the mountainside, he spotted a rectangular orange object about the size of a shoebox. Bending down, he was astonished to realize that it was the cockpit voice recorder, damaged but intact. “So often you hear about them searching for three or four days for the box, and this was recovered in less than half an hour,” he told me one recent morning as he led me around the crash site, his first time back in nearly nine months. “It was all a bit of luck.”

Beaud radioed his colleagues with news of what he'd found, and within hours, a team of forensic specialists flew the device to Marseille and then on to Paris. For the next several days, Beaud and others searching for clues in the debris remained at the site, and Beaud even spent a night camped among the carnage. As he lay in a tent in the blackness, surrounded by utter silence, he thought of the passengers and their last minutes of terror. “I could imagine what they went through,” he recalled, “and it was hard to sleep.”

But the mystery of what brought down Flight 9525 wouldn't be solved on the mountain. Within 36 hours of Beaud's discovery, French authorities would analyze the voice recorder—and discern the almost incomprehensible truth behind the crash.

Part I: Before

Two hours before Beaud was lowered onto that hillside, the Germanwings gate staff at Terminal 2 in Barcelona's El Prat Airport began the boarding process for Flight 9525. Martyn Matthews, a 50-year-old engineer for the German auto-parts giant Huf, was among the first of the 144 passengers to board, taking a seat at the front of the plane. Matthews, a soccer fan, hiker, and father of two grown children, was heading home via Düsseldorf to his wife of 25 years in Wolverhampton, a city in the British Midlands. Maria Radner, a prominent opera singer who had just finished a gig performing Richard Wagner's Siegfried in Barcelona, sat in row 19, along with her partner, Sascha Schenk, an insurance broker, and their toddler son, Felix. Sixteen high school students and two teachers from the German town of Haltern am See, exhausted after a weeklong exchange program, filled up the rear rows of the full flight. The students included Lea Drüppel, a gregarious 15-year-old with dreams of being a professional musician and stage actress, and her best friend and next-door neighbor, Caja Westermann, also 15.

The Airbus sat at the gate for 26 minutes past its scheduled departure time of 9:35, then taxied to the runway and took off, rising over the city and banking gently toward the Mediterranean Sea. From the cockpit, Captain Patrick Sondenheimer, a veteran with 6,000 hours in the air, apologized for the delay and promised to try to make up the lost time en route. At one point, Sondenheimer mentioned to his co-pilot, Andreas Lubitz, that he forgot to go to the bathroom before they boarded. “Go any time,” Lubitz told him. At 10:27, after the Airbus had reached its cruising altitude of 38,000 feet, Sondenheimer told Lubitz to begin preparing for landing (it was only a two-hour flight), a routine that included gauging the fuel levels, ensuring that the flaps and landing gear were working, and checking the latest airport and weather information. Lubitz's response was cryptic. “Hopefully,” he said. “We'll see.” It's unclear if Sondenheimer noted his co-pilot's odd language, but he said nothing in response. A minute later, Sondenheimer pushed his seat back, opened the cockpit door, closed it behind him, and ducked into the lavatory. It was 10:30 A.M.

Francis Pellier / French Interior Minstry/AP Photo

Somehow, amid a vast field of debris scattered over a mountainside in the French Alps, the cockpit voice recorder was located less than a half hour after the first of the first responders arrived on the scene.

Andreas Günter Lubitz, known to his family as Andy, had always wanted to fly. He grew up in Montabaur, a prosperous town of 12,000 located midway between Düsseldorf and Frankfurt, in the green hills of southwest Germany. The firstborn son of Günter Lubitz, a banker, and his wife, Ursula, a piano teacher who played the organ at church, he was a quiet child with a crew cut and a sweet smile. Passionate from boyhood about becoming a pilot, Lubitz papered his bedroom with posters from Airbus, Boeing, and Lufthansa. He also became an expert glider pilot, spending many weekends at a flying club in Montabaur. An advertisement that Lufthansa placed on the back of his high school yearbook asked: “Do you want to make your dream of flying a reality?” To Lubitz, a disciplined student who was voted “third-most orderly” of his graduating class of 2007, the answer was yes. He applied to join the company's flight academy straight out of high school and in 2008 was among the 5 percent of applicants accepted into the program.

That September, Lubitz joined 200 candidates at the Lufthansa Flight Training Pilot School in Bremen, in northern Germany, where students study aviation theory for a year before putting it into practice at flight-training school in Arizona. But in November, just a couple of months into the program, he dropped out and returned home. Two months after that, a Montabaur psychiatrist diagnosed Lubitz as suffering from a “deep depressive episode,” with thoughts of suicide, and treated him with intense psychotherapy and with Cipralex and mirtazapine, two powerful antidepressants. The psychiatrist (whose name is protected by German privacy law) attributed the collapse in part to “modified living conditions,” meaning the move to Bremen and the separation from his parents and younger brother. Lubitz's family would tell investigators that he had developed in his new environment an “unfounded fear of failure.” The breakdown was accompanied, according to case files generated by a prosecutor in Düsseldorf*,* by tinnitus, a near-constant ringing in his ears—a symptom that is often associated with depression.

Lubitz spent nine months in the psychiatrist's care. In July 2009, only six months into the treatment, the doctor declared that “a considerable remission had been obtained” with the meds and recommended in a letter to German aviation officials that Lubitz be allowed to resume his training in Bremen: “Patient alert and mentally fully oriented, with no retentivity or memory disorders. Mr. Lubitz completely recovered, there is not any residuum remained. The treatment has been finished.” Yet the doctor continued to treat Lubitz—and prescribe him powerful drugs—through October, three months after having assured officials that Lubitz had fully recovered. German aviation officials took several more months to restore Lubitz's student pilot's license and his fit-to-fly medical certificate, amending them with the designation SIC, for “specific regular examination.” This notation would stay on Lubitz's record. Any further psychiatric treatment for depression, any more meds, would result in his automatic grounding. As Lubitz was surely aware, this would almost certainly mean the end of his flying career.

Martyn Matthews sat up front, steps from where the captain tried to crowbar the cockpit open. Courtesy of Sharon Matthews

Lubitz completed his Bremen training in early 2010 and then, in preparation for the four-month session at the Lufthansa-owned flight school in Arizona, filled out a student-pilot document required by the U.S. Federal Aviation Administration. Asked on the form if he had ever been diagnosed with “mental disorders of any sort, depression, anxiety, etc.,” Lubitz lied. He ticked off “no,” then left blank the space below, in which he was required to detail any medical treatment he had received over the past three years. But the lie was caught. Four days after Lubitz submitted the form to the FAA, an aviation doctor in Germany who vets documents for the U.S. agency spotted Lubitz's false statement and reported it. Lying on an FAA application can land a pilot in jail for perjury or get him permanently barred from flying. In Lubitz's case, though, the falsehood delayed, but didn't derail, the process. “We are unable to establish your eligibility to hold an airman medical certificate at this time,” responded an FAA official. “Due to your history of reactive depression, please submit a current detailed status report from your prescribing physician.” In other words, Lubitz was given a second shot—and this time, he came clean, admitting his history of depression and complying with the request for a doctor's report. Apparently this was enough to satisfy the authorities on both sides of the Atlantic. Weeks later, he was on his way to Arizona.