RECIFE, Brazil (AP) -- 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.
"This is the material that we've seen that really was part of the plane," Cardoso said.
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 to "avoid errors in interpretation" 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.
Aviation officials have said the crash investigation is increasingly focused on whether external instruments may have iced over, confusing speed sensors and leading computers to set the plane's speed too fast or slow - a potentially deadly mistake in severe turbulence.
Airbus has said the French agency investigating the crash found the doomed flight received inconsistent airspeed readings by different instruments as it struggled with severe turbulence in a massive thunderstorm.
The Air France memo says the company will finish replacing the instruments - known as Pitot tubes - in "coming weeks." It does not say when the replacement process started and the company declined to comment on the advisory, saying it was meant for pilots only.
The L-shaped metal Pitot tubes jut from the wing or fuselage of a plane, and are heated to prevent icing. The pressure of air entering the tubes lets sensors measure the speed and angle of flight.
An iced-over, blocked or malfunctioning Pitot tube could cause an airspeed sensor to fail, and lead the computer controlling the plane to accelerate or decelerate in a potentially dangerous fashion.
Questions about speed sensors are only one of many factors investigators are considering. Automatic transmissions from the plane showed a chain of computer system failures that indicate the plane broke apart in midair.
The cause may be hidden on "black box" voice and data recorders that could lie miles deep on the ocean floor.
But with satellites blocked by thick clouds and heavy rain reducing visibility in an ocean full of floating garbage, searchers have so far been unable to find any confirmed wreckage from the plane.
"Until you have the piece of debris on board and it is conclusively identified as coming from the doomed aircraft, everything is speculation," Goelz said. "The searchers are back to square one with only a best guess at where to start their search for the recorders."
Goelz said searchers need to quickly target an area and drop listening devices into the ocean in hopes of picking up the recorders' signals, which may only continue for 30 days.
French officials have warned the black boxes - and the answer to why the jet crashed - may never be found.
On Thursday, European plane maker Airbus sent an advisory to all operators of the A330 reminding them of how to handle the plane in conditions similar to those experienced by Flight 447.
Goelz said that advisory and the Air France memo about replacing flight-speed instruments "certainly raises questions about whether the Pitot tubes, which are critical to the pilot's understanding of what's going on, were operating effectively."
Meteorologists say the Air France jet entered an unusual storm with 100 mph updrafts that created turbulence while sucking water up from the ocean to high altitude, where it quickly froze in minus-40 degree temperatures.
France is sending a submarine to the area to try to detect signals from the black boxes, said military spokesman Christophe Prazuck. The Emeraude will arrive next week, he said.
Brazil's Air Force flew relatives of victims Friday to the search command post in the northeastern city of Recife, where they toured the operation and asked questions but decided not to speak with the media and then returned to Rio de Janeiro. Recife has a large air force base where debris and any human remains would be brought.
Greg Keller reported from Paris. Bradley Brooks in Rio de Janeiro; Alan Clendenning in Sao Paulo; Emma Vandore, Laurence Joan-Grange, Jamey Keaten and Philippe Sotto in Paris; Daniel Woolls in Madrid; and Slobodan Lekic in Modena, Italy, also contributed to this report.