UPDATE 3-French crash investigation advancing-official

* Conditions hamper crash search

* Investigator urges patience

(Recasts, sources confirm French access to autopsies)

By Tim Hepher and Peter Murphy

PARIS/SAO PAULO, June 17 (Reuters) - French investigators are getting closer to understanding the cause of an Air France crash that killed 228 people, but difficult search conditions in the Atlantic Ocean are hampering the process, France's chief air disaster investigator said on Wednesday.

The Airbus 330 crashed into the sea en route from Rio de Janeiro to Paris on June 1, killing all on aboard.

The French military is using a mini-submarine to search the sea for the "black box" data and voice recorders which may offer clues to the cause.

"We are getting a little closer to our goal but don't ask me what the percentage of hope is," Paul-Louis Arslanian, head of the French BEA agency in charge of investigating the crash, told a news conference without giving details of its progress.

He said the remote location and uneven surface of the ocean where the search was focused posed one of the biggest challenges in the air crash investigation. The uneven ocean floor means the wreckage could lie anywhere between a depth of 1 km (0.6 miles) and 4 km (2.5 miles).