Investigators failed to extract information from a black box of the plane that crashed last month in Mali.
According to the French investigators entrusted with the examination, the voice recorder aboard the unfortunate Air Algerie flight, made in the 1990s, was equipped with a magnetic tape, which got broken and crumpled. However, the investigators said, the malfunction was likely not a result of the impact but rather a technical glitch.
Nevertheless, the investigators were unable to decipher the conversation between the pilots in the minutes before the disaster that killed all 116 passengers aboard the McDonnell Douglas MD-83 aircraft on 24 July this year.
The investigators tried to repair the tape but the conversation was still unintelligible.
"There is sound on the tape but it is unintelligible," Remi Jouty, president of France's BEA air accident investigation office told a news conference.
"The device seemed to be recording but we don't yet know why it did not work, except that this was not a result of the crash itself," he told reporters, adding that first indications were that it was a "simple technical problem".
Magnetic tape technology is no longer used in modern aircraft as it has been replaced by digital recorders.
The investigators still believe bad weather, which was reportedly in the area on the night of the disaster, caused the crash. However, they have not completely ruled out other explanations.
"We're trying to avoid overly hasty theories," Jouty said.
The pilots had asked for permission to alter their route because of a storm as they flew north after taking off from the capital of Burkina Faso, Ouagadougou.
The jet made a detour to steer around the storm cell, but as it did so it gradually lost some height and speed, according to data from the other black box presented at the BEA's headquarters on Thursday.
After broadly resuming its original course, the aircraft abruptly turned back on itself to the left and entered a corkscrew-shaped descent.
It hit the ground at high speed and the impact was "extremely violent", Jouty said.
The strong concentration of debris in one spot on the ground leads experts to believe that the plane crashed upon impact rather than disintegrating in the air, Jouty said.
A first report will be published in mid-September, said the head of the Malian investigating committee.