Investigators looking for the Air France plane that disappeared into the Atlantic Ocean in June still cannot explain the crash and need more money and resources to search for the aircraft's data recorders, the man leading the probe said Monday.

"At the moment, we can't explain the accident," said Paul-Louis Arslanian said, head of France's Bureau of Investigation and Analysis, or BEA.

He said a third, more meticulous search for plane debris and the flight recorders, should begin before the end of the year, but could cost tens of millions of euros (dollars).

"We are making progress and will make progress and I'm optimistic, but this will take time," he told journalists in Paris. "It takes a year and a half, being responsible and reasonable, in order to make progress and ensure that we've run through all of the questions."

Investigators still don't know exactly where the Airbus A330 flying from Rio to Paris crashed, killing all 228 people aboard, or what caused the June 1 accident, Arslanian told a gathering of aerospace journalists in Paris.

The search has so far failed to locate the plane's data recorders, without which the full causes of the crash may never be fully known — but investigators don't want to give up.

Arslanian said planemaker Airbus has offered to help fund the efforts, but more commitments are needed.

"We have to mobilize resources. It's not only having promises for money, we need to know who will contribute financially and how," he said.

Arslanian said around 1,000 parts of the plane have so far been recovered from the Atlantic Ocean — including a nearly intact tail, an engine cover, uninflated life jackets, seats and kitchen items.

The BEA, together with an international team of experts, has been studying the data gathered from two phases of research to decide what a third search phase would cost and require. The second phase of the search began after the black boxes stopped emitting signals, about a month after the crash.

The crash site is more than 900 miles (1,450 kilometers) off Brazil's northeastern coast.

A preliminary report into the crash said the plane hit the ocean intact and belly first at a high rate of speed. Investigators have announced no signs of explosion or terrorism.

The Brazilian authorities have yet to send detailed information about the autopsies on the 50 bodies that have been recovered, although the BEA is working with general information obtained from French authorities, Arslanian said.

He said the agency has not issued a recommendation to airlines over speed measuring equipment because he doesn't have the evidence to justify it.

Since the accident, European air safety regulators have told world airlines to replace hundreds of air speed sensors of the type fitted to the crashed plane.

A series of automatic messages sent by the plane point to a malfunction of the external speed monitors, known as Pitot tubes, which some experts think may have iced over and given false speed readings to the Air France plane's computers as it ran into a turbulent thunderstorm.

"If I had thought it was important to make a recommendation, I would have done it," Arslanian said.

Problems with Pitots are not unusual, but the malfunction doesn't normally last longer than a few seconds, he said.

The BEA is also working with authorities from Yemen and Comoros to find the causes of the Yemenia Airways crash last June, which killed 152 people.

The recently recovered black boxes from that crash should arrive in France on Monday with the Comoran investigators and will be examined by the BEA, Arslanian said.