The officials promised a thorough investigation but said the circumstances were very difficult.
Flight AF 447 was heading from Rio to Paris with 228 people on board on Monday when it was lost over the ocean.
Debris has been spotted 650km (400 miles) off Brazil’s coast and navy vessels are converging on the area.
Brazilian and French officials said there was no doubt the debris was from the missing plane.
A Brazilian air force plane found more, larger items of debris on Wednesday about 90km (55 miles) south of where other wreckage was spotted, a spokesman said.
“Several objects spread over a 5km (three-mile) radius, including an apparently metallic object 7m (23ft) in diameter and a fuel slick” were discovered, Col Jorge Amaral said.
The French civil aviation officials, at a news conference in Paris, said they hoped there would be an initial report by the end of June.
The officials, headed by Paul-Louis Arslanian, chief of the French civil aviation ministry’s bureau of investigation, said there had appeared to be no problems with the flight before take-off.
Mr Arslanian said there would be no speculation and that it was “essential we check and verify everything”.
He said: “This catastrophe – which is the worst that our country has witnessed in terms of aviation, took place in a very difficult region… so the investigation will not be easy… but we are not giving up.”
Mr Arslanian said the exact time of the accident was not known, nor whether the chief pilot was at the controls.