French President Nicolas Sarkozy and his predecessor, Jacques Chirac, took part Wednesday at a multi-denominational service at Notre Dame Cathedral in Paris commemorating the 228 victims of Monday's crash of an Air France jetliner, according to dpa. Sarkozy was accompanied to the service by his wife, Carla Bruni-Sarkozy, as well as Prime Minister Francois Fillon. Also attending were Air France executives and employees as well as relatives of some of the victims of Monday's accident. "We pray for the spouses, the parents and children who have lost their loved ones," said the archbishop of Paris, Cardinal Andre Vingt-Trois at the service. "And also for the personnel of Air France, who will continue to fly." For the duration of the service, 228 candles burned before the cathedral's altar, for each of the victim's of France's worst-ever air disaster. There were 72 French nationals, 60 Brazilians and 26 Germans among the plane's 216 passengers and crew of 12. The memorial service began as Brazilian air force planes continued to find pieces of wreckage believed to be from the Airbus A330-200 that crashed into the Atlantic while traveling from Rio de Janeiro to Paris. Air Force spokesman Jorge Amaral said in Brasilia that among the 10 newly-discovered objects there was a large piece about 7 meters long. In addition, oil and petrol traces were discovered along a 20-kilometer swathe of the Atlantic Ocean approximately 1,200 kilometers from the Brazilian coast. On Tuesday, Brazilian Defence Minister Nelson Jobim said there was "no doubt" that debris found in the area belonged to Air France flight 447. A Brazilian ship, the Grajau, has reached the area and will be seeking to retrieve the debris discovered by the planes, Admiral Savio Nogueira said. A total of five Brazilian navy ships are on the way to the area. Earlier Wednesday, an investigation into the cause of the world's worst commercial air disaster since 2001 was formally opened. Officials of France's Office of Accident Investigations and Analyses (BEA), which is heading the inquiry, said Wednesday that the search for the cause of the crash would be long and difficult. "This aviation catastrophe is the worst this country has ever suffered," BEA director Paul-Louis Arslanian told journalists in Paris. "We can not allow ourselves to speculate. We must verify everything." Arslanian said the accident occurred in the middle of the Atlantic, where the waters are very deep and the seafloor is very mountainous. This makes locating the sunken aircraft extremely difficult. Arslanian was very clear about the first priority of the investigation. "We have to find the black boxes," he said. Each plane carries two black boxes, a flight data recorder and a cockpit voice recorder, and they are essential in helping investigators in the painstaking search for the cause of a plane crash. "I don't know what we will find. But we will immediately communicate everything we learn, as soon as it is clear to us," Arslanian said. The BEA said that a first report on its investigation would be issued at the end of June. Meanwhile, the search for the fuselage of the aircraft continued. Captain Christophe Prazuck, of the French army chief of staff, said that Wednesday would be "a day of transition" in that search. "We will move from an air operation covering a large zone to a naval operation over a restricted zone," he was quoted as saying by the online edition of the daily Le Figaro. The merchant ships and navy vessels in the area will gather the debris floating on the surface before submarines are deployed in the coming weeks to search for the wreckage itself, he said. In addition, an AWACS surveillance plane will carry out a "cartography" of the debris in an attempt to determine the precise place where the aircraft crashed into the sea, to help in the eventual recovery of the plane's black boxes, Prazuck said.