German police have seized possessions belonging to a co-pilot who apparently crashed his plane in the French Alps killing all 150 people on board, as they investigate his possible motives.
They said they had found a significant clue, according to media reports.
Data from the plane's voice recorder suggest Mr Lubitz had deliberately started a descent while the pilot was locked out of the cockpit.
The Barcelona-Duesseldorf flight crashed on Tuesday.
Several airlines have now pledged to change their rules to ensure at least two crew members are present in the cockpit at all times.
The revelations by the German police come after officers searched Mr Lubitz's flat in Duesseldorf and the house the 27-year-old shared with his parents in Montabaur, north of Frankfurt, late on Thursday.
A number of items were removed - including boxes and a computer - from the two properties.
"We have found something which will now be taken for tests. We cannot say what it is at the moment but it may be a very significant clue to what has happened," the Daily Mail quoted police spokesman Markus Niesczery as saying.
However, police said the discovery was not a suicide note.
There were also unconfirmed reports in the German media that Mr Lubitz had suffered from depression.
Meanwhile, German government officials said Mr Lubitz was not known to the country's security services.
What happened in the final half hour?
Earlier, Carsten Spohr, the head of Lufthansa, the German carrier that owns Germanwings, said the co-pilot had undergone intensive training and "was 100% fit to fly without any caveats".
Mr Spohr said Mr Lubitz's training had been interrupted for several months six years ago, but did not say why.
The training was resumed after "the suitability of the candidate was re-established", he said.
Members of the Westerwald flight club, where the co-pilot was a member, expressed their shock at the revelation.
"Andres was a very nice young man, who did his training here. He was part of the club," Peter Reucker said.
"[He was] funny, sometimes a bit quiet, but apart from that a young man like many others that we have here. He integrated well.
"I'm absolutely speechless. I have no explanation for this," Mr Reucker added.
On Thursday, Marseille prosecutor Brice Robin said the co-pilot appeared to want to "destroy the plane".
Citing information from the recovered "black box" voice recorder, Mr Robin said Mr Lubitz was alone in the cockpit just before the crash.
Mr Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.
Air traffic controllers made repeated attempts to contact the aircraft, the prosecutor added, but to no avail.
Passengers were not aware of the impending crash "until the very last moment" when screams could be heard, Mr Robin said, adding that they died instantly.