NEW : Oil slick may argue against an in-flight explosion , Brazilian official says . NEW : Determining where plane broke up will be difficult , Brazil aviation expert says . Air France flight disappeared over the Atlantic Ocean after taking off from Paris . But Brazilian President Luiz Inacio Lula da Silva expressed optimism that the flight data recorders would be found . Memorial for the victims of Flight 415 takes place in U.S. at Notre Dame .
NEW : Oil slick may argue against an in-flight explosion , Brazilian official says . NEW : Determining where plane broke up will be difficult , Brazil aviation expert says . Air France flight disappeared over the Atlantic Ocean after taking off from Paris . Memorial for the victims of Flight 415 takes place in U.S. at Notre Dame .