The recovery effort to locate remains and debris from Germanwings Flight 4U 9525, already hampered by the rocky terrain of the French Alps, will be even more treacherous on Wednesday with snow forecast to fall on the crash site, CNN reported. While the plane’s black box has been recovered -- officials said it was damaged but in working condition -- investigators still have not found the Airbus A320’s flight data recorder or the remains of the 150 people on board believed to have died in Tuesday’s crash.

A cause for the crash of Flight 4U 9525, which took off from Barcelona, Spain, and was en route to Düsseldorf, Germany, when it crashed in the south of France Tuesday morning, has yet to be determined. “We are entering a phase of criminal investigation which will be long. It has to be done in a very precise way, with meticulous detail in order to find the bodies, to allow for their identification, and to give them back to their families, while of course collecting the evidence which will help advance the investigation,” French Interior Ministry spokesman Pierre-Henry Brandet said Wednesday, according to Euronews.

What investigators do know is that Flight 4U 9525 experienced a steep, sudden drop to 8,000 feet before tumbling further to 6,000 feet, when it lost contact with air traffic control. It crashed shortly after radio contact was lost.

The terrain of the French Alps, where peaks can extend as much as 1.9 miles, presents challenges for investigators trying to reach the site, according to CNN. A crash in the mountains also means the debris field may be vast, and pieces of debris may be tiny, making them hard to locate and recover.

A Germanwings official said Tuesday that investigators with Lufthansa, the German airline that owns low-cost carrier Germanwings, would be taken to the crash site by helicopter. The search was stopped late Tuesday due to bad weather and poor visibility.