The pilot of a chartered plane carrying a Brazilian football team which crashed in the Andes told air traffic controllers he had run out of fuel before the plane came down, according to a leaked recording.
In sometimes chaotic audiotape from the air traffic tower, the pilot of the British-built jet could be heard repeatedly requesting permission to land due to a "total electric failure" and lack of fuel, before slamming into a mountainside late on Monday.
The crash killed all but six of the 77 people on board, including members of Brazil's Chapecoense side, which was travelling to the Copa Sudamericana final.
A controller could be heard giving instructions as the aircraft lost speed and altitude about eight miles from Medellin airport. Just before going silent, the pilot said he was flying at an altitude of 9,000ft. The recordings, obtained by several Colombian media outlets, seemed to confirm the accounts of a surviving flight attendant and a pilot flying nearby who overheard the frantic pleas from the doomed airliner.
These, along with the lack of an explosion upon impact, point to a rare case of fuel running out as a cause of the crash. Experts say the jet was flying at its maximum range.
For now, authorities are avoiding singling out any one cause of the crash. A full investigation is expected to take months and will review everything from the 17-year-old aircraft's flight and maintenance history to the voice and instruments data in the black boxes retrieved at the crash site on a muddy hillside.
Alfredo Bocanegra, head of Colombia's aviation agency, said that while evidence initially pointed to an electrical problem, the possibility the crash was caused by lack of fuel has not been ruled out.
Before being taken offline, the website of LaMia, the Bolivian-based charter company, said the Avro RJ85 jetliner's maximum range was 1,600 nautical miles - just under the distance between Medellin and Santa Cruz, Bolivia, where the flight originated carrying close to full passenger capacity.
It is also possible the pilot dumped fuel, or a lack of fuel was caused by a leak or some other, unexplained reason.
When asked whether the plane should have attempted such a long journey, Bocanegro said: "If this is confirmed by the investigators it would be a very painful because it stems from negligence."
One key piece to unlocking the mystery could come from Ximena Sanchez, a Bolivian flight attendant who survived the crash and told rescuers the plane had run out of fuel moments before the crash. Investigators expect to interview her on Wednesday at the clinic near Medellin where she is recovering.
Investigators also want to speak to Juan Sebastian Upegui, the co-pilot on an Avianca commercial flight who was in contact with air traffic controllers near Medellin's Jose Maria Cordova airport at the time the chartered plane went down.
Another clue is the crash site itself, where no traces of fuel have been found. Often planes go up in a ball of flames upon impact but one reason six passengers survived was because the plane did not explode.
John Cox, a retired airline pilot and CEO of Florida-based Safety Operating Systems, said the aircraft's amount of fuel deserves a careful look.
He said: "The aeroplane was being flight planned right to its maximum. Right there it says that even if everything goes well they are not going to have a large amount of fuel when they arrive.
"I don't understand how they could do the flight non-stop with the fuel requirements that the regulations stipulate."