BERLIN (AP) -- Police have searched the homes of co-pilot Andreas Lubitz in two German cities in search of an explanation for why he may have crashed a passenger plane into the French Alps, killing all 150 people on board. German tabloid Bild reported Friday that Lubitz had a "serious depressive episode" six years ago and that a medical problem was noted in aviation records. The Federal Aviation Office couldn't immediately be reached for comment. French investigators believe the 27-year-old locked himself inside the cockpit and then intentionally smashed the Germanwings plane into a mountainside. A spokeswoman for Duesseldorf police, Susanna Heusgen, said "no crucial piece of evidence has been found yet" after the searches in Duesseldorf and Montabaur. Duesseldorf prosecutors say they plan to release an update later Friday.