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Members of Brazil's military search for debris from an Air France jet over the Atlantic Ocean on Wednesday. Rescuers have still found no signs of life, but have spotted debris, including an airplane seat, a fuel slick, an orange lifevest and pieces of white debris. ((AP Photo/Brazil Defence Ministry))

Hundreds of mourners gathered at Notre Dame Cathedral in Paris on Wednesday to remember the victims of the crashed Air France Flight 447, as investigators announced there was little optimism that the black boxes would ever be found.

Families of the 228 people aboard the Airbus A330 when it fell off the radar screens on Sunday night en route from Rio de Janeiro to Paris attended the private ceremony with officials from Air France and French President Nicolas Sarkozy.

As the mourners gathered on one side of the Atlantic Ocean, Brazilian officials announced search planes have found a debris field that includes what appears to be a seven-metre piece of the plane.

Search planes had spotted a five-kilometre trail of wreckage — including an orange life-vest, a small white piece of metal and what is believed to be an airliner seat — on Tuesday more than 640 kilometres northeast of the archipelago of Fernando de Noronha, off Brazil's coast.

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People enter Notre-Dame Cathedral in Paris for an ecumenical church service for relatives and families of the passengers of Air France's flight 447. ((Bob Edme/Associated Press))

The latest wreckage, which was spotted about 90 km south of the other debris field, contains about 10 metal objects and an oil slick that has spread about 20 km, said Brazilian air force Col. Jorge Amaral.

Officials have said there are no signs of survivors or the remains of passengers.

Bells toll 228 times

The bells at Notre Dame tolled 228 times for the 216 passengers and 12 crew on Flight 447.

Among the passengers were vacationers, newlyweds, a group of business people returning from a trip they'd been sent on as a reward for their work, an 11-year-old boy returning to England alone, and Brad Clemes, 49, a Coca-Cola executive who was born and raised in Guelph, Ont., and lived in Belgium.

President Nicolas Sarkozy met with the families privately following the memorial to provide them with an update on the investigation.

Sarkozy has reportedly told the families that arrangements will be made so they can fly over the debris field if that is something they feel they need to do for closure.

Brazil is taking the lead in the search for wreckage while France will take charge of the crash investigation.

No signals from black boxes detected

The French investigators provided their first update on the investigation at a press conference at an airport north of Paris on Wednesday.

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A slick that is believed to be from the fuel of Air France Flight 447 is seen from the window of a Brazilian air force plane patrolling the crash area in the Atlantic Ocean roughly 1,200 km northeast of Recife on Tuesday. ((Reuters/Brazilian Air Force/Handout))

Paul-Louis Arslanian, head of France's accident investigation agency, said he is not optimistic the black boxes from the plane will ever be found.

Finding the black boxes, which are likely under thousands of metres of water on the mountainous ocean floor, will require searchers to narrow their efforts to a very limited range, Arslanian said.

Planes searching the suspected crash site have not picked up any signals from the black boxes, which are designed to transmit at a low frequency for up to 30 days, he added.

"We may be unable to find the black boxes due to circumstance and logistics," he said.

Aviation experts have said black boxes are virtually indestructible and designed to withstand a high impact crash and underwater pressure.

But Arslanian warned that even if they are recovered, investigators may find the data is useless to the investigation or has been destroyed.

Early Wednesday, French military officials said that there was no doubt the wreckage was the remains of the missing plane.

Investigators have not yet formed an opinion on whether the plane broke up in the air or on impact with the ocean, Arslanian said.

Air France said it received a bomb threat for a flight between Buenos Aires and Paris on May 27, but a check of the aircraft proved negative and the flight proceeded.

France's Defence Ministry said there are no signs so far of terrorism in connection with the tragedy.

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Debris from Air France Flight 447 has been found about 650 kilometres northeast of the archipelago of Fernando de Noronha. ((CBC))

A full investigation will be conducted even if the black boxes are not found, he said. It's unclear how long the process would take.

Investigators are studying the four-year-old jetliner's maintenance records and there are no indications it had any problems before takeoff, said Arslanian.

It is also not yet clear if the chief pilot on the flight was at the controls when the plane crashed into the ocean, he added. Pilots on long-haul flights often take turns at the controls to remain alert.

Preliminary report by month's end

The accident agency will submit a preliminary report by the end of June, said Alain Bouillard, who is leading the probe.

French military spokesman Christophe Prazuck said France is moving the mission to recover the wreckage into a naval operation, which will include submarines to find the plane's two black boxes.

Some experts have said the remote area of the ocean, its depth and the weather in the region at this time of year could make the recovery effort one of the most difficult since the exploration of the sunken Titanic.

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Canadian Brad Clemes, a 49-year-old father of two adult sons, was originally from Guelph, but had been living with his wife in Belgium and working for the past 14 years as an executive for Coca-Cola. ((Courtesy Clemes family))

In addition to French military aircraft that are patrolling the area, France has dispatched a research ship equipped with unmanned submarines that can reach a depth of 6,000 metres. The submersibles will be used to recover wreckage settling beneath the ocean's surface.

Four Brazilian navy ships carrying recovery equipment and divers are also en route to the wreckage.

A U.S. navy P-3C Orion surveillance plane — which can fly low over the ocean for 12 hours at a time and has radar and sonar designed to track submarines underwater — and a French AWACS radar plane are joining the operation.

If there are no survivors, as expected, it will be the world's deadliest commercial airline disaster since Nov. 12, 2001, when an American Airlines jetliner crashed in the New York City borough of Queens during a flight to the Dominican Republic, killing 265 people.

Aviation experts have said determining the cause of the crash will be challenging if the flight-data recorders are not recovered.

"I cannot rule out the possibility that we might end up with a finding that is relatively unsatisfactory in terms of certainty," Arslanian said.

Air France received an automatic message from the plane indicating an electrical circuit malfunction about four hours into the flight, at 10 p.m. ET on Sunday. The message came shortly after the flight crossed "through a thunderous zone with strong turbulence," Air France officials said.

Towering Atlantic storms

Some officials have speculated the plane may have been struck by lightning, but aviation experts have said that should not have been enough to bring down the aircraft.

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Eithne Walls, 29, an Irish doctor, was among the 228 people who were on Air France Flight 447 when it disappeared from radar screens.

Other potential causes include shifting winds and hail from towering thunderheads, a massive mechanical failure or a combination of other factors.

Meteorologists have speculated the aircraft might have faced winds as strong as 160 km/h created from a series of thunderstorms along the flight path.

Towering Atlantic storms are common this time of year near the equator — an area known as the intertropical convergence zone.

With files from The Associated Press