*Update at 8:11 a.m. ET: A French prosecutor announced this morning that the co-pilot seems to have deliberately crashed the plane. Details below. *

One of the pilots of the Germanwings flight that crashed in the French Alps Tuesday morning was locked out of the cockpit as the plane went down, The New York Times reports. The co-pilot, identified as 28-year-old German citizen Andreas Lubitz, had control of the flight, and according to French prosecutor Brice Robin, seems to have deliberately initiated the plane's descent.

“The interpretation that for us, investigators, is the most plausible is that the co-pilot, through a deliberate omission, refused to open the door of the cockpit to the captain and activated the loss of altitude button for a reason that we are totally unaware of, but that can be interpreted as an intention to destroy this aircraft,” Robin said.

According to an investigator with evidence from the recovered cockpit voice recorder, the pilots' conversation early in the flight was natural, and eventually one stepped out of the cockpit, then couldn't get back in.

“The guy outside is knocking lightly on the door and there is no answer,” the investigator told the Times. “And then he hits the door stronger and no answer. There is never an answer."

"You can hear he is trying to smash the door down.”

On Thursday, Robin said that in the final moments before impact, the voice recorder also picked up a faint sound from within the cockpit: co-pilot Lubitz's steady breath. It was not the breathing of a person in distress, Robin said.

It's stunning evidence in the ongoing investigation into what caused the crash of Flight 9525 with 150 people on board. Authorities have recovered the two black boxes, an arduous task that required searching remote, mountainous terrain accessible only by air, but one of them, the flight data recorder, is so badly damaged that determining exactly what happened will be a difficult task.

A Punishing Task

One day after the plane went down en route to Dusseldorf from Barcelona with 150 people aboard, investigators had confirmed there were no survivors and begun the process of tagging and analyzing the debris field. Flight 9525 began descending shortly after reaching its cruising attitude of 38,000 feet, and air traffic control lost all contact at about 6,000 feet. The plane appears to have hit the ground at high speed.

Investigators struggled to retrieve data from the cockpit voice recorder, which was found Tuesday, not long after the search began, but eventually managed. The flight data recorder is in far worse condition. Investigators found the crucial device, which constantly monitors hundreds of parameters like airspeed, heading, attitude, altitude, and the position of flight control surfaces, Wednesday. But the memory chip, which contains all that data, is missing. If it wasn't destroyed, it's somewhere amid the debris scattered over 2 square kilometers of unforgivingly rugged terrain.

“It’s really like a puzzle, putting it back together," says Steven Meyers, an aircraft accident investigator and head of DVI Aviation. Except lots of pieces are missing, or scorched, or shattered, and there's no image on the box to give you guidelines.

The highly unusual nature of the crash makes the black box evidence especially important. The Airbus was flying a routine route in good weather, with an experienced crew operating an aircraft with an excellent safety record. Yet the plane mysteriously began a steady descent shortly after reaching its cruising altitude, dropping more than 30,000 feet in about 10 minutes.

Adding to the mystery of one pilot being locked out is the fact no one aboard issued a distress call. In January this year, one Delta pilot was kept out of the cockpit by a malfunctioning door. In that case, air traffic control was informed (as were the passengers), and the co-pilot safely landed the plane. But the common rule in aviation is "aviate, navigate, communicate," in that order. In other words, issuing a distress call is a low priority compared to actually flying the plane.

Investigators do have other data to work with. The radar flight path can provide clues as to how the aircraft was behaving, and if the pilots were trying to correct for a problem with the aircraft. The pattern and state of debris can reveal some of the story. Calculating the trajectory and impact angle of various parts can indicate the position of the jet in its final moments, for example. Close examination of wreckage can provide clues. Scorching or soot suggest a fire. Looking at whatever’s left of the cabin and cockpit will yield more information—if, for example, the oxygen masks were deployed.

Here, the usefulness of that information's limited. The plane essentially disintegrated, and the smaller pieces will be harder to identify and won't reveal as much, says Meyers. The mountainous terrain makes debris difficult to access and collect, making what is a laborious process under the best circumstances downright daunting.

The Missing Black Box

The biggest questions are why the plane descended, why no one aboard said anything, and what was going on with the pilots. Did the pilot left in the cockpit mean to bring it down? Was he conscious? Was it some kind of autopilot malfunction or mixed up setting? The flight data recorder, with all its information on how the plane was being flown, could reveal the answers, or at least be used to corroborate or disprove any theories.

The black boxes, according to the BEA, the French equivalent of the National Transportation Safety Board, are certified to withstand the kind of brutal conditions that plane crashes often entail. Acceleration up to 3,400 g. Temperature up to 2,012° F for an hour. A full month under 6 miles of water. "These characteristics," the BEA says, "allow investigators to successfully access data from the flight recorders after an accident in almost 100 percent of cases."

Almost 100 percent. It is not yet known how fast the Airbus A320 was going when it the mountain terrain, but it was fast enough to destroy the aircraft. "If you've got a high impact, high fragmentation, [the black boxes are] not gonna survive," Meyers says. "Everything has a threshold for failure."

The case of Air France 447, which crashed over the Atlantic in 2009, shows how missing large chunks of information hinder an investigation. Weeks after the crash, the BEA issued an interim report "without any analysis and—given the continuing absence of wreckage, the flight recorders, radar tracks and direct testimony—without any description of the circumstances of the accident." Basically, it said nothing.

It wasn't until the Airbus A330's black boxes and much of the wreckage was found nearly two years later, that a follow-up offered real conclusions and substantial findings, along with recommendations for avoiding similar crashes in the future.