Information collected by investigators suggests the co-pilot who was in control of the Germanwings airplane when it crashed Tuesday was acting deliberately, Marseille prosecutor Brice Robin said. All 150 people on board were killed.
The co-pilot apparently "wanted to destroy the aircraft," Marseille prosecutor Brice Robin said.
Audio from the cockpit voice recorder revealed that the captain had left the cockpit to use the restroom, the prosecutor said, and could not get back in. The audio reveals the captain was banging on the door of the cockpit, but the co-pilot did not take any action to let him in.
"It was when he was alone that he manipulated the buttons of the flight monitoring system to activate the descent of the aircraft," Robin said. "The action can only be voluntary."
But why, in an era of heightened aviation security, was a pilot allowed to be solo in the cockpit, with tragic consequences?
Geoffrey Thomas, editor-in-chief of airlineratings.com, said the cockpit door has three positions, citing Airbus A320 captains and the aircraft manual -- unlocked, normal and locked.
The door would usually be in the normal position, but if the reports are true, it would appear that after one of the pilots left the cockpit, it was switched to the locked position. Doing so prevents the other pilot from using a keypad and emergency code to get in, he said.
CNN aviation analyst David Soucie said that American carriers require that any pilot leaving the flight deck be replaced by a flight attendant, so that two people remain in the cockpit at all times, but that's not the case with all airlines internationally.
"The authorities and regulations stipulate that a pilot can be in the cockpit alone," Lufthansa spokesman Waber Joerg said. "It is recommended that this time be kept to a minimum. We comply with all German and European aviation authorities."
In the wake of the Germanwings crash, several airlines announced they were amending their policies to mandate that two crew members be in the cockpit at all times.
Post 9/11 security
Peter Goelz, former managing director of the U.S. National Transportation Safety Board, said aviation security working groups had focused on changing protocols after the September 11 attacks to prevent further hijackings.
"They talked extensively about whether we should give a random member of the flight crew ... an ability to enter the cockpit," he told CNN.
"But then they decided that ... from a security standpoint you couldn't guarantee that outsiders wouldn't learn that."
But aviation analyst Scott Hamilton said it appeared that such a practice could have made a crucial difference had it been implemented on Flight 9525.
"Had a flight attendant gone in, she could have let the pilot back in," he said.
Goelz said the post 9/11 improvements instead focused on armoring the cockpit and cockpit door lock to prevent intrusions, and "procedures that the pilots take to fly the airplane in a different way if hijackings are attempted."
"Those doors are built to withstand hand grenade blasts -- you can't knock them down by beating them with your fists," said Desmond Ross, principal of DRA Professional Aviation Services.
Aviation analysts explained that the cockpit door would automatically lock once it closed. The pilot outside the cockpit could then re-enter by entering a code on the door.
"That ordinarily would work," Hamilton said. "But if there's what's called an override thrown -- basically a double lock -- he can't get back in."
Ross said the override was a "simple switch" on the control panel with three positions: lock, normal, unlock.
"There's a CCTV camera outside the door so the guy on the flight deck can see whether it really is his companion coming back in again or whether it's someone else trying to break in."
If the pilot who had left the cockpit "was being denied access after punching in the correct code, it meant that somebody was seriously trying to keep him out," he said.
Thomas of airlineratings.com said he understood from Airbus A320 pilots and the aircraft's manual that there was no way for anyone outside the cockpit to open the door once the override lock had been activated.
"It would appear that someone has deliberately locked that door (on Flight 9525)," he said.
This made the explanation of pilot suicide -- of which there had been eight cases in 40 years -- a more likely explanation, he said.
The latest crash has also raised the question of whether cameras should be used in cockpits.
In an age of ubiquitous camera surveillance, pilots have stubbornly resisted the intrusion of video cameras on flight decks.
Pilot and aviation analyst Jim Tilmon said while many passengers would view it as a welcome innovation, "from the pilot's point of view, it's not just an inconvenience or an intrusion, it's just kind of unnecessary."
"There are other kinds of ways to get the information," he said.
"I'm not interested in knowing whether or not the pilot's picking his nose."
Pilot and author Karlene Petitt said she's also opposed to cameras. "If someone was to do something up there and there was a camera, you'd (only) need to get up and put your coat over it or a hat," she said.
"There's so many better ways that we could spend that money."
Soucie, the CNN aviation analyst, said the smartest innovation would be to focus on streaming live data back to ground control during a flight.
"It feels like we're in archaeology trying to find these 'black boxes,' " he said, referring to the data recorders.