Brazil's defence ministry has released photos of an oil slick on the sea
The hunt for clues as to the fate of an Air France jet lost in an Atlantic storm has intensified with the arrival of the first Brazilian navy vessel.
French aviation officials say they may never find the jet's flight data recorders, which could be under waters 3,700m (12,100ft) deep.
Aerial searches have not reported bodies but Brazil's air force has seen more, and larger, debris.
The jet was carrying 228 people from Rio to Paris when it was lost.
Wreckage seen from the air was spread over a 5km area and included a metallic object 7m in diameter and an oil slick stretching over 20km (13 miles).
Brazilian Defence Minister Nelson Jobim said the presence of oil slicks could eventually rule out the possibility of a fire or explosion on the plane, Reuters news agency reports.
Meanwhile, an unconfirmed report in a Brazilian newspaper has detailed what it says is the final series of critical failures in the systems on board the Airbus 330.
'Fuselage or tail'
With each passing day the search operation in the Atlantic has been growing in size, the BBC's Gary Duffy reports from Sao Paulo.
Eleven aircraft have now been deployed to the area, about 1,100km (690 miles) north-east of Brazil's coast, and the first of five Brazilian navy vessels arrived on Wednesday.
The latest wreckage is said to have been spotted some distance to the south of where the first pieces of debris were discovered on Tuesday.
"We are considering this 7m piece to be part of the plane, possibly part of the side, a piece of steel," said Brazilian air force spokesman Col Jorge Amaral.
"It could be part of the fuselage or the tail."
France's minister responsible for transportation, Jean-Louis Borloo, warned that "black boxes" had never been recovered at such depths.
"The sea currents are powerful that far down," he said.
Paul Louis Arslanian, head of France's air accident investigation agency, said: "We cannot rule out that we will not find the flight recorders."
While the flight recorders were designed to send homing signals for up to 30 days after hitting water, there was no guarantee they even survived the impact, he added.
Cascade of messages
According to the report in Brazil's Estado de Sao Paulo newspaper, citing an unidentified Air France source, the doomed airliner's pilot first sent a message that he was entering thick black clouds of a type normally associated with violent winds and lightning.
Ten minutes later a series of electronic messages were sent from the plane indicating that the autopilot had disengaged and that a computer on board had switched to an alternative power system.
The controls needed to keep the aircraft stable had also been damaged, the newspaper report says, and an alarm sounded, suggesting the situation was becoming increasingly grave.
This cascade of messages ended with one pointing to a loss of air pressure and electrical failure.
The French authorities who are leading the investigation into the causes of the crash declined to comment.
A US aviation safety expert, Bill Voss of the Flight Safety Foundation, said the Brazilian newspaper account strongly suggested the plane had broken apart in the air without explaining why.
"This is the documentation of the seconds when control was lost and the aircraft started to break up in air," he told the Associated Press.
It is clear, our correspondent says, that the only definitive explanation will come with the recovery of as much debris as possible and, crucially, the flight data recorders.