The German pilot who deliberately flew his airliner into a mountainside last year had struggled with learning to fly and had failed a key test of his skills during training in the U.S., according to FBI interviews with his flight instructors.

Andreas Lubitz was promoted anyway. But his training difficulties were one more "red flag" that should have caused Lufthansa and the airline's Arizona flight school to take a closer look and discover his history of depression, asserted attorneys representing families of crash victims.

Lubitz was a co-pilot for Germanwings, a regional airline owned by Lufthansa, when he locked Flight 9524's captain out of the cockpit and set the plane on a collision course with a mountain in the French Alps last year. All 144 passengers and six crew members, including Lubitz, were killed.

One instructor, Juergen Theerkorn, described Lubitz as "not an ace pilot," and said he failed one flight test because of a "situational awareness issue." In aviation, loss of situational awareness usually means a pilot becomes absorbed in something and loses track of what else is happening with the plane.

Another instructor, Scott Nickell, told the FBI that Lubitz lacked "procedural knowledge" and had trouble with splitting his attention between instruments inside the plane and watching what was happening outside. But while Lubitz struggled with training, he would achieve passing scores enabling him to continue the program, Nickell said.

Lubitz failed one of five check rides, which are important tests of a pilot's flying skills, and one of 67 training flights, Matthias Kippenberg, president and CEO of the Airline Training Center Arizona, told the FBI. However, Kippenberg dismissed the failures as unremarkable, saying students are given the opportunity to retake the tests. Only 1 or 2 percent of students fail to be promoted, he said.

The FBI conducted the interviews a week after the March 24, 2015, crash. Summaries were only recently released by prosecutors in Germany, according to attorneys with Kriendler & Kriendler in New York, who are representing the families in a lawsuit against the flight school. The lawyers provided copies to The Associated Press.

Lufthansa spokeswoman Christina Semmel declined to comment "due to the ongoing legal proceedings." The flight school referred calls to Lufthansa.

Officials for Lufthansa and the flight school didn't immediately reply to requests for comment.

An investigation has revealed that Lubitz was being treated for a relapse of severe depression and suicidal tendencies but had hid the information from Germanwings.

Germany's strict patient privacy laws didn't allow doctors to share medical information with an employer without the patient's permission.

Lubitz had had a previous bout of depression in 2008 and had informed Lufthansa, taking a leave of absence two months after starting ground school training in Germany. He was allowed to resume training ten months later after providing a statement from his doctor that he had recovered.

Lubitz was originally scheduled to begin his training at the flight school in Arizona in September 2009, but was rescheduled for September 2010. He didn't actually start until November. Lufthansa told the school in an email that the delay was due to "a long illness," Sherri Harwood, the school's administrative services manager, told the FBI.

The FBI summaries don't contain a copy of that email, so it's not known whether Lufthansa told the school the nature of Lubitz's illness, said Brian Alexander, one of the attorneys representing the families.

The FBI interviews show that flight school officials "acknowledge knowing (Lubitz) struggled in training, had a long illness and was delayed for over a year," Alexander said. "They also admit he failed a check ride due to a loss of situational awareness, which may very well have been caused by the very same anxiety and severe depression which were symptoms of his mental health disorder."

It remains unclear what specific information the school had about Lubitz' medical condition. But If the school had checked, Alexander said, it might have learned that German authorities had twice turned down applications from Lubitz for a pilot medical certificate because of his history of depression before issuing him a certificate in July 2009. That certificate stated it would become invalid if he had a relapse.

In the U.S., the Federal Aviation Administration also initially declined to grant Lubitz a student pilot medical certificate because he said on his application that he hadn't been treated for any mental disorders, and he failed to list doctors who had treated him as required. After a medical examiner working for the FAA in Germany caught the discrepancy, Lubitz refiled a corrected application.

The FAA could have refused to issue the certificate because Lubitz lied on the application, but he was allowed to provide a statement from his doctor that he was fit to fly and that medications for depression had been discontinued.

John Goglia, an aviation safety expert and former National Transportation Safety Board member, agreed with attorneys that Lubitz's struggles were a warning that should have caused the school to look closer, although "not a bright red one." It's not unusual for students to fail a single check ride, he said.

The school's washout rate of only 1 or 2 percent seems low, he said.