Recife, Brazil ? Days after Air France Flight 447 vanished, an intensive international effort has failed to recover any confirmed wreckage and concern grew Friday about whether searchers were even looking in the right place.

Air France, meanwhile, told its pilots in a memo obtained by The Associated Press that it is replacing instruments that affect flight speed in all its bigger jets. Investigators have focused on the equipment’s possible role in the disaster.

Brazilian officials first reported Tuesday that military pilots had spotted wreckage from Flight 447 scattered across the ocean’s surface, but pieces pulled out Thursday turned out to be unrelated to the plane.

Air Force Brig. Gen. Ramon Cardoso insisted Friday that at least some of the debris spotted from the air — an airplane seat, a slick of kerosene and other pieces — are from the plane that vanished Sunday with 228 people on board. The Brazilian air force also distributed images pinpointing where the material was found.

But officials said extremely poor visibility has hampered efforts to guide ships to the spot where the debris was sighted, and France’s Transportation Minister Dominique Bussereau said his own country’s searchers have found no signs of the Airbus A330.

“French authorities have been saying for several days that we have to be extremely prudent,” Bussereau told France’s RTL radio. “Our planes and naval ships have seen nothing.”

A French Defense Ministry official, speaking on condition of anonymity because he was not authorized to speak publicly on the matter, also questioned the Brazilian claims, saying French teams “cannot precisely confirm the zone where the plane went down.”

A statement issued jointly by Brazil’s Air Force and Navy said that from now on any debris recovered in the area will only be divulged after being “positively identified” by Air France as coming from the ill-fated flight.