The German pilot who deliberately flew an airliner filled with passengers into a mountainside last year was trained in Arizona. The FBI now said there were many warning signs during his training.

Through interviews with flight instructors, the agency has learned Andreas Lubitz struggled to learn to fly. He also failed a key test of his skills at the Airline Training Center Arizona in Goodyear, but Lubitz was promoted anyway.

Attorneys representing families of the crash victims said these red flags should have caused Lufthansa and the flight school to take a closer look at Lubitz.

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 to crash into the French Alps last year. All 144 passengers and six crew members, were killed, including Lubitz.

The Associated Press contributed to this report.