WASHINGTON (AP) — 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 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.

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.