At 10:01 a.m. on Mar. 24, passengers in Barcelona, Spain boarded a seemingly harmless airplane flight to Düsseldorf, Germany. The passengers thought they had nothing to worry about–the chances of a plane crash are astronomically small. However, what nobody realized is that the co-pilot may have had another agenda than to just get them safely to their destination.

27-year-old Andreas Lubitz was assisting pilot 34-year-old Patrick Sondenheimer on Germanwings flight 9525. Investigators have found evidence of Lubitz lacking proper mental health and hiding it from his employer. Antidepressants were found in his Düsseldorf home along with multiple ripped notes saying that he was unfit for work. According to CNN, Lubitz’s employer has said that there “was never any indication Lubitz was depressed, and that if he went to a doctor on his own he would have been required to self-report if he had been deemed unfit to fly.”

When Sondenheimer left the cockpit to use the restroom, Lubitz locked him out. “The guy outside is knocking lightly on the door, and there is no answer,” an investigator who heard the audio tape of the pilots’ conversations mentioned. “And then he hits the door stronger, and no answer. There is never an answer.” Sondenheimer continued to knock on the door until he had no choice but to try and smash it in. Meanwhile, his co-pilot guided the jet to descend at more than 3,000 feet per minute from its cruising altitude of 38,000 feet. And by noon that day, the plane had hit the French Alps and crashed, killing everyone on board.

Investigators have mostly been looking into Lubitz’s motive, but also the lives of the passengers, including two Americans, German teenagers on a school trip and their teacher, and many more. Despite being so far away, our hearts go out to the families of all 150 victims.