The French accident investigation agency said on Wednesday it was not hopeful that the black box of the missing Air France airliner would be found.

Speaking at the first news conference since the disappearance of the Air France flight AF447 from Rio to Paris on Monday, Paul-Louis Arslanian, the director of France's air safety investigation agency, said he was "not optimistic" that the box would be found in the "deep sea and mountainous area."

A relative of the missing airliner's victims leaves the Rio de Janeiro International Airport escorted by guards in Rio de Janeiro, Brazil, June 1, 2009. An Air France airliner with 228 people onboard missing over the Atlantic Ocean after its takeoff from Rio de Janeiro on Sunday was probably hit by lightning and suffered an electrics failure while flying through an Atlantic storm, said Air France on Monday. (Xinhua/Song Weiwei)

Four teams have been sent to an area in the Atlantic Ocean to investigate the case. The first team aims to locate the exact position of the crash; the second is seeking information relating to the cause of the crash and the history of the passenger jet; the third is trying to find out the operation of the plane, and the fourth is focusing on the operation of the equipment.

Investigators are working with Air France, Airbus and meteorologists to determine what caused Air France Flight 447 to crash with 228 people on board into the Atlantic Ocean off the Brazilian coast on Monday.

The agency is expected to have an initial report on the disaster ready by the end of June, Arslanian said.

Describing the crash as the most serious in French aviation history, Arslanian said France will take charge of the investigations according to related international laws.

Asked what might have caused the crash, Arslanian said there were too many uncertainties regarding information obtained so far, plus the complexity of the sea area where the plane crashed, which all added to the difficulty of the investigations.

So far they could not determine whether the plane broke up in the air or upon impact with the sea, he said, warning that investigations were likely to last for a long time.

The studies of the plane's maintenance and other records showed no evidence that it had problems before takeoff from Rio de Janeiro on Sunday at 7 p.m. (2200 GMT), Arslanian said.

However, he declined to comment on an earlier Air France statement that the plane might have been hit by lightning.

Regarding the black box, Arslanian said he was "not optimistic" that rescuers could recover it.

"I am not so optimistic. It is not only deep, it is also mountainous," he said. "We might find ourselves blocked at some point by the lack of material elements."

However, he noted that if rescuers do not find the black box, investigators should be prepared to continue the probe through other sources.

The black box, which contain voice and data recorders, is built to last 30 days at about 6,000 meters underwater.

The person in charge of the investigation, Alain Bouillard also told the press conference on Wednesday the accident investigation agency would submit its first preliminary report by the end of June.

While France has taken charge of the crash investigation, Brazil has been leading the search for wreckage, and at present five vessels are en route to the scene.

Besides searching the surface, remotely controlled submersible craft will have to be used to recover wreckage and the black box. The task is expected to be exceedingly challenging as the water depth in the area is about 7,000 meters.

So far no black box has ever been tracked from that deep.

France has sent a research ship equipped with unmanned submarines that can explore as deep as 6,000 meters, according to the French military.

In another development, a Brazilian air force spokesman on Wednesday said bigger debris, including what appears to be a seven-meter-long chunk of the plane has been spotted, though he said that they bore no obvious markings.

Before that, spotter jets had found an aircraft seat, an orange life vest, and some engine oil floating on the sea surface, which were later identified to be from the plane.

No signs of life have so far been found.

Editor: Yan