PARIS >> The co-pilot of Germanwings Flight 9525 appeared to have practiced a controlled descent on his flight into Barcelona just two hours before he intentionally crashed the A320 jet into a mountainside on the return flight to Duesseldorf, air accident investigators said Wednesday.

Authorities are still puzzling over why co-pilot Andreas Lubitz, who had suffered from suicidal tendencies and depression in the past, sent the Barcelona-to-Duesseldorf flight straight into the French Alps on March 24, killing all 150 people on board.

This latest development about an earlier flight appears to support the assumption that the crash was not only deliberate but also premeditated, and raises questions about all of the flights where Lubitz was in the cockpit.

Lubitz seemed to be toying with the plane’s settings on a March 24 flight from Duesseldorf to Barcelona, programming it for sharp descent multiple times in a 4 1/2-minute period while the pilot was out of the cockpit before resetting the controls, France’s BEA investigation agency said in an interim report on the crash.

Prosecutors have said that Lubitz intentionally locked the pilot out of the cockpit and crashed the plane on its return flight to Duesseldorf.

Wednesday’s 30-page report said the same crew was aboard both flights — and the pilot appeared to have left the cockpit during the earlier flight as well, for about 4 ½ minutes.

On the first flight into Barcelona, shortly after the pilot left, the "selected altitude" of the flight changed repeatedly, including several times being set as low as 100 feet (30 meters) above the ground. The report says co-pilot Lubitz also put the engines on idle, which gives the plane the ability to quickly descend.

It would be highly unusual for a pilot to repeatedly set a plane for such a low altitude for no apparent reason. However, the report says that Lubitz did so while he was being asked by air traffic controllers to bring the plane down gradually from 35,000 feet to 21,000 feet for its scheduled descent to Barcelona.

A chart released by the BEA showed the plane didn’t actually descend sharply while Lubitz was repeatedly adjusting the settings, so the passengers and crew might not have noticed any change. The BEA report did not make it clear whether all the steps needed for the repeated descents were taken by Lubitz.

"The captain didn’t realize at all, because the co-pilot’s tests during the outgoing flight took place during a normal, preprogrammed descent and it never had an impact on the plane’s trajectory," said Remi Jouty, the director of BEA.

Aviation experts said the findings were clearly unusual.

"The process of going up and down with the selected altitude is not normal — but I can’t tell you what was going on in his head," said Antoine Amar, a top official in France’s main pilots union SNPL and an Air France pilot who has flown the A320, the same type of plane in the Germanwings crash.

Germanwings’ parent company Lufthansa technically could have known about Lubitz’ apparent rehearsal on the outbound flight, but only if it had looked at the flight data in the short period while the plane was unloading and loading passengers in Barcelona.

Germanwings said it "welcomed any new information that can help clarify what happened" but said it would not comment on the BEA report because of the ongoing investigation. It would also not comment on whether other flights involving Lubitz were now under review. The airline referred all questions to French prosecutors and the French civil aviation authority.

Amar, the French union official, suggested there was little that air authorities could have done between the two flight legs. Authorities would need access to the cockpit recorders and the data downloaded to know whether Lubitz had repeatedly chosen a 100-foot altitude during the flight.

Duesseldorf prosecutors who are investigating the co-pilot said they don’t know if any passengers on the first flight into Barcelona were even questioned.

Lufthansa spokesman Helmut Tolksdorf said Wednesday by phone that the airline had not yet had time to analyze the new details released by the French authorities and planned no immediate comment.

The BEA report didn’t analyze why Lubitz repeatedly tried to descend the plane going into Barcelona and did not say if it would be looking into other flights where Lubitz was in the cockpit. However, the agency said it is continuing to look at the "systemic failings that may have led to this accident or similar events."

The investigators are focusing on "the current balance between medical confidentiality and flight safety," a reference to whether German rules on patient privacy may have prevented Lufthansa from being fully aware of Lubitz’ mental health issues.

The BEA said investigators are also looking at "compromises" made on security after the Sept. 11 attacks in the U.S., notably on cockpit door locking systems meant to protect pilots from terrorists. Since the Germanwings crash, several airlines have imposed rules requiring two people in the cockpit at all times.

French prosecutors are conducting a separate criminal investigation into the crash.

It remains unclear why Lubitz would have wanted to crash the plane. German authorities say he researched suicide methods and cockpit door security online in the week before the crash.

Duesseldorf prosecutors said they would not comment on the BEA report until they have finished evaluating Lubitz’s computer and other documents.

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Angela Charlton in Paris, David Rising in Berlin and Scott Mayerowitz contributed to this report.