French officials said on Wednesday they may never discover why an Air France aircraft crashed into the Atlantic killing 228 people and cautioned they might not even find the plane’s black boxes on the ocean floor.

Officials in Brazil said search teams had spotted four more clusters of debris about 90 km south of the first wreckage discovered on Tuesday in the middle of the Atlantic.

Brazilian and French ships were headed to the area, some 745 miles northeast of Brazil’s coastal city Recife hoping to retrieve as much of the wreckage as possible. France dispatched a mini-submarine that can explore to a depth of 6,000 metres and will try to locate the Airbus’s flight data and voice recorders, which should shed light on a crash that has puzzled aviation experts.

But Paul Louis Arslanian, the head of France’s air accident investigation agency, said he was not sure that the black boxes would be recovered and said the probe might prove frustrating.

“I cannot rule out the possibility that we might end up with a finding that is relatively unsatisfactory in terms of certainty.”

He revealed few new elements, confirming only that the crew had sent a radio message reporting turbulence as it headed toward the equator.

“For now, there is no indication to suggest that the plane had a problem before its take off,” he told a news conference.

Air France confirmed on Wednesday that it had received an anonymous phone call warning that a bomb was on a flight leaving Buenos Aires on May 27, four days before the Rio crash.