PARIS, France (CNN) -- French officials -- who said there is now no hope that anyone survived the crash of an Air France jet that plunged into the Atlantic -- said Wednesday they may never find the doomed jets' flight data recorders.
The Airbus 330, with 216 passengers and 12 crew members on board, disappeared from radar screens, some three hours after it took off from Rio de Janeiro in Brazil en route to Paris.
A memorial for the victims of Flight AF 447, which included 61 people from France, 58 Brazilians, 26 Germans and people from 29 other countries, will take place at the Notre Dame Cathedral in Paris at 10 a.m. ET (4 p.m. GMT). Brazil has declared three days of mourning.
Air France CEO Pierre-Henri Gourgeon said aircraft encountered heavy turbulence about 02:15 a.m. local time Monday (10:15 p.m. ET Sunday). At that point, the plane's automatic system initiated a four-minute exchange of messages to the company's maintenance computers, indicating "several pieces of aircraft equipment were at fault or had broken down."
The jet, which was flying at 35,000 feet and at 521 mph, also sent a warning that it had lost pressure, the Brazilian air force said. Its last known contact occurred at 02:33 a.m., the Brazilian air force spokesman added. Map of Flight AF 447's flightpath »
"We need time to reach the recorders," said Paul-Louis Arslanian, head of France's Accident Investigation Bureau.
The recorders give off a locator signal that lasts for up to 30 days, and the French government has sent a research vessel with a deep-diving submersible on board to the area where the plane's debris was found.
French officials said weather conditions at the site -- believed to be a major factor in the crash -- remain "extremely difficult" and the depth of the Atlantic near the sites where wreckage was found is around 7,000 meters (21,000 feet).
"Even in history ... recorders from time to time were found after the 30 days. But I'm not so optimistic," said Arslanian at a press conference Wednesday. "It's not only deep it's also very mountainous at that place of the ocean."
Arslanian said answers on what caused the crash could take a long time.
"It could be long, we can not do with 80 percent understanding," said Arslanian. "This catastrophe is the worst that our country has known in our country's air history."
He said there appeared to be no problems with Flight AF 447 before take-off, but that everything had to be "checked and verified."