The man, named as Andreas Lubitz, deliberately "accelerated the descent" of the plane that crashed earlier this week, authorities said.
As details continue to unfold about the crew and the final moments of flight 4U9525, a family friend of the co-pilot has claimed in a German newspaper that he was suffering from mental health problems.
A mother of a schoolmate of Mr Lubitz told the Frankfurter Allgemeine Zeitung he had told her daughter that he had taken a break from his pilot training because he was suffering from depression.
Earlier, French prosecutors said the aircraft's Captain - who is not being blamed for the crash - had left the cockpit and was then prevented from reentering by Mr Lubitz.
Speaking at a press conference in Marseille, prosecutor Brice Robin said the passengers were only aware of the incident moments before the crash, when screams could be heard on the cockpit's audio recording.
"We hear the pilot ask the co-pilot to take control of the plane and we hear at the same time the sound of a seat moving backwards and the sound of a door closing," Mr Robin told reporters.
"At that moment, the co-pilot is controlling the plane by himself.
"While he is alone, the co-pilot presses the buttons of the flight monitoring system to put into action the descent of the aeroplane.
Prosecutor Robin explained that it was impossible to lock the door by accident and it was impossible to put the plane into such a steep dive by accident
Reports suggested there was a "violent struggle" between the two airmen, who were "shouting at each other" moments before impact.
The co-pilot is believed to have been alive at the moment the plane hit the side of the mountain at 500mph.
Reports also emerged overnight that the co-pilot was locked out of the cockpit and tried to smash down the door.
The two airmen were said to have fought to open and close the door, with a chair used at one stage to try and prevent access to the cockpit.
Evidence from the plane's voice recorder revealed that one of the pilots desperately tried to break down the cockpit door in the moments before the horror crash.
Mr Robin added the investigation was currently an involuntary homicide but that was likely to change to a voluntary homicide – effectively a mass murder probe.
He said Mr Lubitz, who was a German national and perfectly able to fly the plane on his own, was alone at the controls when the plane crashed and had activated the descent button.
He added: “Data from the cockpit suggests he was breathing normally and had not passed out, he was conscious.
We currently have an involuntary homicide but given that a voluntary action appears to have downed this plane that is likely to change.
“To go from 12,000 meters to 2000 meters you need to give a particular lever several deliberate turns.
“There is no doubt that this was a voluntary action. To refuse entrance to the cockpit and start to reduce altitude at 1000 meters per minute as if he was coming into land.
“At no point have we considered this a terrorist attack."
The pilots had been having "very smooth, very cool" conversation during the flight from Barcelona to Dusseldorf, a senior military official said
But then one left the cockpit and was unable to get back in, the New York Times reported.
“The guy outside is knocking lightly on the door and there is no answer,” an unnamed investigator revealed.
“And then he hits the door stronger and no answer. There is never an answer.”
He added: “You can hear he is trying to smash the door down."
It is not yet known why the pilot left the cockpit or what happened to the one left in control of the plane.
Germanwings has so far refused to confirm the names of the pilots, with the company tweeting that it wanted to "honour their privacy".
The newspaper reports the captain was the father of two children and had flown for over ten years for Lufthansa and Germanwings.
The German pilots union said they will not comment on the tragedy until after a press conference scheduled by the plane's operator for 1.30pm GMT.
Speaking this morning, Lufthansa's CEO, Carsten Spohr, said: "Safety in aviation is not a given, it is something that we have top work hard for every day and every night.
In a video on Twitter, he added: "Something happened that we worked so hard against that it would never hit us.
"Our thoughts and prayers at this very moment are with the relatives of the passengers and crew members that lost their lives."
Four specialist officers from Interpol, the international police organisation, have joined the investigation as French detectives continue to piece together information surrounding the crash.
The worrying development comes after aviation experts expressed fears the mystery crash could have been caused by a lack of cabin oxygen.
Ex-pilot Eric Moody said a major incident must have occurred on board as the pilots were not "thinking straight" after the Airbus A320 plunged into the French Alps at 500mph on Tuesday.
The 73-year-old pilot listed oxygen deprivation, depressurisation - or even "a knife in the throat" - as potential causes of the crash.
Other reports have suggested the cockpit's windscreen gave way, incapacitating the pilots and rendering them unable to send a distress call.
Germanwings flight 4U9525, from Barcelona to Dusseldorf, crashed after an eight-minute rapid descent, killing all 144 passengers and six crew
But while a spokesman for the French air investigation bureau, the BEA, said that "usable" material had been extracted from the cockpit voice recorder covering the entire flight, he would not give details of conversations, if any, between the pilots.
Asked about reports that the second black box - the flight data recorder which indicates how an aircraft's systems were working - had been found but was too badly damaged to be of use, the spokesman said this was not the case.
"We have not located the second black box," he said.
