BERLIN (Bloomberg) - Investigators are assuming the Germanwings pilot who appeared to intentionally crash a plane into the French Alps on Tuesday, taking 149 passengers and crew to their deaths, was going through a "personal life crisis", German paper Bild Zeitung reported, citing unidentified security authorities.

Andreas Lubitz was having relationship difficulties with his girlfriend, which may have played a role in his apparent decision to initiate the descent into the mountainside at high speed, the newspaper reported, without identifying the woman.

The first officer had to repeat some stages of flight school because of depression and was occasionally listed as "unfit to fly" during his training at Deutsche Lufthansa AG's flight school in Arizona, Bild said.

Lufthansa is the parent company of low-cost carrier Germanwings. The medical records of personnel are bound by German confidentiality laws, a Lufthansa spokesman said on Friday, declining to comment further on the report.

Lubitz, who started his pilot training in 2008, took leave for "several months" at one point, Lufthansa chief executive Carsten Spohr said on Thursday, declining to elaborate. Marseille prosecutors, who are leading the investigation, said Lubitz was aged 28, while some German reports say he was 27.

Mr Spohr called the incident a "tragic, isolated case".

Police on Thursday searched Lubitz's apartment in Dusseldorf and the house he grew up in in the town of Montabaur, about 140km away, for personal documents that might provide clues as to why he crashed the plane, said the prosecutors' office in Dusseldorf, which has opened an official investigation.

Police have removed several items in their searches. "We have found things and taken them," Mr Andreas Czogalla, a spokesman for Dusseldorf police, said by phone. "We can't comment on the results, the evaluation is ongoing."

Lubitz's reason for deliberately initiating the fateful descent of Germanwings flight 9525 while the captain was locked out of the cockpit isn't known, said Mr Brice Robin, a French prosecutor handling the investigation.

Lubitz's file at the federal office of civil aviation and his pilot's license had a special code on it that means he needed special medical checks, Bild reported.

All pilots are routinely reassessed and Lubitz was deemed fit to fly, Mr Spohr said on Thursday. The co-pilot had psychological treatment for 1 1/2 years, Bild reported.

Crash investigators are rushing to find the onboard data recorder to confirm prosecutors' suspicions that the co-pilot steered the jet into a mountain after locking the captain out of the cockpit.

Audio files from the flight deck revealed that the co-pilot began descending after the captain stepped into the cabin, then denied him re-entry, prosecutor Mr Robin said. Except for his breathing, the co-pilot stayed silent until the plane slammed into a slope at full speed, Mr Robin said.

The findings from the voice recorder on the Airbus A320 suggest that the crash, the worst-ever accident for Lufthansa and Germanwings, was deliberate rather than due to a technical fault. The data unit, the second half of the airliner's so-called black boxes, is important because it tracks the changes made by the crew to the controls.

"The most plausible interpretation is that the co-pilot voluntarily refused to open the door for the captain and activated the button controlling the plane's altitude for a reason we don't know," Mr Robin said on Thursday at a press briefing. The action "can be interpreted as a desire to destroy the plane."

Flight 9525 was en route to Dusseldorf from Barcelona when the crash occurred.

The voice and data recorders are housed in specially reinforced cases to protect against the forces of a crash and painted orange to ensure visibility amid the wreckage. The single-aisle A320 from Airbus Group NV was largely pulverized by the impact, which Robin estimated occurred at 700 kilometers an hour.

Data gathered by tracking service Flightradar24 showed Flight 9525's autopilot initially was programmed to 30,000 feet (9,100m) as the jet climbed, then reset to 32,000 feet and finally to its 38,000-foot cruising altitude, co-founder Mikael Robertsson said. Then the autopilot was manually reset to 96 feet, he said.

"After four minutes we see someone playing with the autopilot and changed it to the lowest setting," Mr Robertsson said in an interview, adding that Flightradar24 has turned over the information to investigators.