Dusseldorf, Germany (CNN) Analysis of a tablet device belonging to Germanwings Flight 9525 co-pilot Andreas Lubitz shows he researched suicide methods on the Internet in the days leading up to the crash, a German prosecutor said Thursday.

Dusseldorf prosecutor Christoph Kumpa said that on one day Lubitz also "searched for several minutes with search terms relating to cockpit doors and their security measures."

Police analysis of the correspondence and search history on the device, retrieved from Lubitz's Dusseldorf apartment, demonstrated that the co-pilot used it from March 16 to March 23, Kumpa said.

The search history was not deleted and also revealed searches concerning medical treatment, the prosecutor said.

Lubitz is suspected of deliberately bringing down Germanwings Flight 9525 in the French Alps on March 24, killing all 150 on board. Investigators have since focused on his health as they try to establish his motivation.

Noting he's made a criminal request to German authorities but is for now conducting his own investigation, the prosecutor in Marseille, France, said he is tasked with an involuntary homicide investigation. But prosecutor Brice Robin noted that Lubitz made voluntary actions -- such as guiding the plane toward the mountain and reducing its speed to prevent alarms from going off -- and was "alive and conscious" to the very end.

A European official government official with detailed knowledge of the investigation said that Lubitz's actions amount to"premeditated murder."

While cautioning that there are still many holes in understanding Lubitz's motivation, the disclosures about his Internet searches show that he planned to do what he was going to do, according to this official.

Second 'black box' found

As authorities try to figure out what was on Lubitz's electronic devices, they got another big break about what was happening inside Airbus 320 that went down -- its flight data recorder.

The jetliner's cockpit voice recorder was located shortly after the plane crashed. Now, investigators have both "black boxes," as the devices are called, and the details that they might provide.

A female police officer digging by hand for clothes in a ravine that been searched previously found the flight data recorder on Thursday afternoon about 8 inches (20 centimeters) below the surface, Robin told reporters.

Normally white with florescent orange, this discovered recorder lived up to its name as a black box because fire had darkened it with ashes. Even with this damage, the Marseille prosecutor said that investigators should be able to get useful information out of it.

"We will be able to identify the speed, the altitude and the way the pilot acted ... which will be critical," said Robin.

Prosecutor: 150 sets of DNA recovered

The voice data recorder is one of many items uncovered at the crash site in the southern French Alps.

Authorities have found 470 personnel effects there, according to Robin. That number includes 40 cell phones, though all those were badly damaged. Robin cast doubt that any useful information could be retrieved from those phones, given their condition.

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This is consistent with French officials' claims Wednesday insisting that two publications, German daily Bild and French Paris Match, were wrong to report that cell phone video showed the harrowing final seconds from on board the flight.

More importantly, investigators have isolated 150 different sets of DNA -- a number that corresponds with the number of people on Flight 9525. Still, the Marseille prosecutor cautioned, "It doesn't mean we have identified 150 victims. We need to compare (the recovered DNA to) DNA from the families and the deceased."

Robin estimated "it will take between three to five weeks, if all goes well" for the passengers' loved ones to get the remains.

But the mourning is already underway.

A memorial stone set up in the village of Le Vernet, the nearest accessible point to the crash site, has become a place of pilgrimage for those with relatives and friends on board the plane.

People in the German town of Haltern recently came together for a memorial service to remember 16 students and two teachers lost in the crash.

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Push to take steps to prevent a repeat

Providing some closures to the families of those passengers and crew members is a top priority for authorities.

So, too, is doing what they can to figure out why this happened and prevent similar tragedies.

There are new calls from aviation experts to develop and deploy enhanced crash avoidance software that could take control of an aircraft away from a pilot and steer it to a safe altitude.

Photos: Germanwings plane crashes in France Photos: Germanwings plane crashes in France A recovery crew works among debris of Germanwings Flight 9525 at the crash site near Seyne-les-Alpes, France, on Friday, April 3. The crash killed all 150 people aboard and has raised questions about the co-pilot's mental state. Hide Caption 1 of 26 Photos: Germanwings plane crashes in France French Gen. David Galtier holds up a picture of the second black box from Germanwings 9525 during a news conference in Marseille, France, on Thursday, April 2. The flight data recorder shows that co-pilot Andreas Lubitz purposely used the controls to speed up the plane's descent, investigators said. Hide Caption 2 of 26 Photos: Germanwings plane crashes in France Rescue workers recover debris from the crash site in the French Alps on Tuesday, March 31. Flight 9525 was traveling from Barcelona, Spain, to Dusseldorf, Germany, when it crashed. Hide Caption 3 of 26 Photos: Germanwings plane crashes in France A helicopter drops rescue workers next to crash debris near Seyne-les-Alpes, France, on Sunday, March 29. Hide Caption 4 of 26 Photos: Germanwings plane crashes in France Forensic experts work near the crash site on Thursday, March 26. Hide Caption 5 of 26 Photos: Germanwings plane crashes in France Journalists stand in front of the Westerwald airfield in Montabaur, Germany, on March 27. Lubitz reportedly learned to fly here. Hide Caption 6 of 26 Photos: Germanwings plane crashes in France Investigators carry a computer from the home of Lubitz's parents in Montabaur, Germany, on Thursday, March 26. Hide Caption 7 of 26 Photos: Germanwings plane crashes in France A police officer stands guard March 26 at an apartment building where Lubitz was thought to have lived in Dusseldorf. Hide Caption 8 of 26 Photos: Germanwings plane crashes in France A helicopter lifts a rescue worker from the crash site on March 26. Hide Caption 9 of 26 Photos: Germanwings plane crashes in France Rescue workers continue to search the site of the crash on March 26. Hide Caption 10 of 26 Photos: Germanwings plane crashes in France Search-and-rescue teams land near the crash site on Wednesday, March 25. Hide Caption 11 of 26 Photos: Germanwings plane crashes in France Debris from the plane is seen along a mountainside on March 25. Hide Caption 12 of 26 Photos: Germanwings plane crashes in France French military personnel move up a mountainside March 25 near Seyne-les-Alpes. Hide Caption 13 of 26 Photos: Germanwings plane crashes in France The cockpit voice recorder of the Germanwings jet appears in this photo provided by the French air accident investigation bureau on March 25. The device is designed to capture all sounds on a plane's flight deck. Hide Caption 14 of 26 Photos: Germanwings plane crashes in France A helicopter comes in to land near Seyne-les-Alpes, the staging ground for search efforts, on March 25. Hide Caption 15 of 26 Photos: Germanwings plane crashes in France Rescue workers and members of the French Gendarmerie gather in Seyne-les-Alpes on Tuesday, March 24, as search-and-rescue teams struggle to reach the remote crash. Hide Caption 16 of 26 Photos: Germanwings plane crashes in France Wreckage is seen at the crash site on March 24. Hide Caption 17 of 26 Photos: Germanwings plane crashes in France Response teams gather in Seyne-les-Alpes on March 24. Hide Caption 18 of 26 Photos: Germanwings plane crashes in France Relatives of the flight's passengers arrive at the airport in Barcelona on March 24. Hide Caption 19 of 26 Photos: Germanwings plane crashes in France People hold hands walking through the Dusseldorf Airport on March 24. Hide Caption 20 of 26 Photos: Germanwings plane crashes in France The arrivals board at the Dusseldorf Airport shows Germanwings Flight 9525 without a status on March 24. Hide Caption 21 of 26 Photos: Germanwings plane crashes in France People arrive at the Dusseldorf Airport on March 24. Hide Caption 22 of 26 Photos: Germanwings plane crashes in France An employee of Swissport, the handling agent of Germanwings flights from Barcelona, speaks by phone at the Barcelona-El Prat Airport on March 24. Hide Caption 23 of 26 Photos: Germanwings plane crashes in France Relatives of people involved in the crash arrive at the Barcelona airport on March 24. Hide Caption 24 of 26 Photos: Germanwings plane crashes in France A man in Madrid looks at a monitor with a map, released from the Flightradar24 website, showing the point where the plane's radar signal went missing. Hide Caption 25 of 26 Photos: Germanwings plane crashes in France This undated file photo shows the Germanwings Airbus A320 that crashed. Germanwings is a low-cost airline owned by the Lufthansa Group. Hide Caption 26 of 26

The idea is not new. In fact more than 10 years ago following 9/11, Airbus, the manufacturer of the doomed aircraft, was working to d‎evelop aircraft crash avoidance software with tech giant Honeywell -- in part to prevent jetliners from being flown into large buildings or mountains. But the project was ultimately scrapped.

Still, there's also a widespread view that the best way to understand what happened in this case is to understand Lubitz.

Source: Lubitz saw multiple doctors

It is becoming increasingly clear to investigators that Lubitz was "very afraid" he would lose his license to fly because of his medical issues, a law enforcement source with detailed knowledge of the investigation told CNN.

It's already emerged that Lubitz had battled depression years before he took the controls of Flight 9525 and that he had concealed from his employer recent medical leave notes saying he was unfit for work.

But the law enforcement source said that after a severe depressive episode in 2009, Lubitz relapsed with severe depression and stress in late 2014.

In the weeks leading up to the crash, Lubitz was shopping doctors, seeing at least five, perhaps as many as six, the source said, as he kept going from one doctor to the next seeking help.

The 27-year-old was having trouble sleeping, and one of the doctors he saw recently was a sleep specialist, the law enforcement source said. Lubitz apparently told some doctors that he was afraid of losing his pilot's license because of his medical issues.

The doctors knew he was a pilot and gave him a "not fit to work" notice, which he was apparently required to give to his employer, even though he didn't, the source said. Officials have found doctors handled the matter the way they were supposed to and found no negligence on their part.

Thinking he would stay home from work based on the sick notes, Lubitz was prescribed a lot of medication, including "heavy depression medicine" that would have been "very heavy" on the body and "critical to a pilot," the source said.

However, the law enforcement source doesn't think Lubitz was using the medicine while working, because investigators interviewed a pilot he flew with the day before who said he was "completely normal" with no problem whatsoever.

Inside Lubitz's apartment, investigators found a couple of notes with only a few words, involving stress and his pilot's license, the source added. He didn't know if Lubitz wrote these down while talking to someone on the phone or wrote the memo to remind himself of something.

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Lubitz told his Lufthansa flight training school in 2009 that he had a "previous episode of severe depression," the German airline confirmed Tuesday. Lufthansa is the parent company of budget airline Germanwings.

Confidentiality issues

German officials announced that a new task force would look at issues including medical procedures for pilots and cockpit door locking mechanisms after the devastating crash.

The cockpit voice recorder revealed that Lubitz had locked the pilot, Patrick Sondenheimer , out of the cockpit before putting the plane into its fatal descent, said Robin, the Marseille prosecutor.

Cockpit doors on planes were strengthened following the events of September 11, 2001, making it impossible for the captain to force his way in.

German Transport Minister Alexander Dobrindt told the news conference Thursday that the task force would prioritize questions around the cockpit door locking mechanism and procedures for checking pilots' medical and psychological well-being.

It would seek to move quickly, he said, but would not make rash decisions.

Klaus-Peter Siegloch, head of the German Aviation Association, emphasized issues around confidentiality when it comes to a pilot's medical records.

"The confidence our pilots have in our medical doctors is of high importance," he said. "I believe if there is a lifting of doctor-patient confidentiality, then possibly pilots will not trust in medical doctors and that will make the situation worse."

The task force will bring together government officials with representatives of the German airline companies, the German Aviation Association and the Federation of German Airlines.