Clues are beginning to emerge in the tragic plane crash that killed Polish President Lech Kaczynski: According to Russian investigators, the accident wasn’t likely caused by mechanical problems, but pilot error. Investigators are now focusing on why the pilot decided to land the jet, despite being warned by air traffic controllers of severe weather conditions. The black box recorder has been recovered, but so far no transcripts of on-board conversations or conversations between the cockpit and the control tower have been released. According to The New York Times, the pilot may have felt pressure to land the plane following a previous incident involving the Polish president: In 2008, Kaczynski reportedly argued with a pilot who was flying his plane to Tbilisi, Georgia, insisting that the pilot land the plane in dangerous conditions. After refusing to heed Kaczynski's orders, the president publicly threatened to punish the pilot. Though he did not face any disciplinary measures, the pilot subsequently suffered from depression. The former president of Poland, Lech Walesa, told a Polish newspaper that it’s not uncommon for pilots to respond to the commands of on-board officials: “If there were any doubts, the leaders were always approached and asked for their decision, and only on this basis were further steps taken."