"What could have destabilized Andreas Lubitz?"

MARSEILLE, France -- European investigators are focusing on the psychological state of a 27-year-old German co-pilot who prosecutors say deliberately flew a plane carrying himself and 149 other people into a mountain, a French police official said Monday.

Returning from a meeting with his counterparts in Germany, judicial police investigator Jean-Pierre Michel told The Associated Press that authorities want to find out "what could have destabilized Andreas Lubitz or driven him to such an act."

Lubitz was the co-pilot of Germanwings Flight 9525 last week that crashed into a French Alps mountain near Le Vernet en route from Barcelona, Spain, to Duesseldorf, Germany, killing everyone on board.

"To have carried out such an act, it's clearly psychological," Michel said.

Authorities are trying to understand what made Lubitz lock his captain out of the cockpit and ignore his pleas to open the door before manually ordering the plane to descend on what should have been a routine flight. To that end, they are speaking with people who knew and worked with Lubitz -- such as co-workers, his employer, his doctors.

CBS News correspondent Allen Pizzey reported Monday that the first hints Lubitz could have been planning something unbelievable on the morning of March 24, were contained in a transcript of the cockpit voice recorder, published Sunday by the German newspaper Bild.

Prompted to go over landing procedures for Dusseldorf, Lubitz reportedly used the words "hopefully," and "we'll see," but then said he was fine to man the controls while the pilot took a bathroom break.

When Captain Patrick Sondheimer came back, however, the cockpit door was locked. He knocks, then bangs harder and shouts "For God's sake open the door," according to the published transcript. Passengers are screaming. More banging, and then an automatic warning goes off in the cockpit: "Terrain. Pull up."

The captain screams: "Open the damn door." Inside the cockpit, Lubitz can be heard breathing normally. A sound like metal scraping a mountaintop is heard, more screams, and then nothing.

French officials have refused to confirm or deny the authenticity of the cockpit voice recorder transcript published by Bild, but the events portrayed do seem in line with details given by a French prosecutor last week.

French officials have refused to confirm or deny news reports suggesting that Lubitz had been on medication for the treatment of depression or other mental issues.

At the remote mountain crash site itself, French authorities were building a road to facilitate access to the site.

CBS Radio News correspondent Elaine Cobbe said the new road wasn't finished Monday, but it was already making the task easier for search teams.

The crews still had to trek the last 40 minutes to the mountainside debris field on foot, but with low cloud cover keeping the police helicopters on the ground Monday, it was the only way up to the site.

Soon the new road will allow investigators to retrieve parts of the destroyed plane. Until Monday, Cobbe said everything has had to be airlifted out, so bigger pieces of the wreckage are still on the mountainside.

Family members of of some of the crash victims were still arriving in the southern French Alps Monday, said Cobbe, anxious to be near to where their loved ones died.

The mayor of Le Vernet, the nearest town, says many relatives ask to visit the crash site itself, but with the search for the second "black box," the flight data recorder and the victims' remains still ongoing, no-one but official search teams is allowed up the mountain.

Germanwings chief operating officer Oliver Wagner was meeting with other victims' relatives in the southeastern city of Marseille. A total of 325 family members have come to France, he told reporters.

Brice Robin, a state prosecutor in the southeastern French city of Marseille, has said that none of the bodies recovered so far have been identified, denying German media reports that Lubitz's body had been found.

Tests on the body of the co-pilot may provide clues about any medical treatment he was receiving. German prosecutors said Friday that Lubitz was hiding an illness and sick notes from a doctor for the day of the crash from his employer.