It lasted only one or two seconds - and was indecipherable to non-expert listeners. It is now known to be an unidentified member of the Chinook crew saying: "Scottish military. Good afternoon. This is Foxtrot Four Juliet Four Zero."
As the brief message played to a silent courtroom, relatives of the dead listened intently.
The inquiry at Paisley, Strathclyde, has been told that the message was sent to the Prestwick air traffic control centre at 5.55pm and 14 seconds on 2 June, 1994 - but was not responded to.
This was less than five minutes before the Chinook crashed with the loss of 29 lives, including Ulster's top police and military intelligence experts. The message was spotted when tape recordings were studied after the crash.
Wing Commander Donald Devine, in charge of the military side of the joint civil and military air traffic control operation at Prestwick, told the inquiry he had no explanation why the message went unheard by the three military staff who were monitoring radio traffic.
A technical investigation had revealed no reason why the message should not have reached their headsets. He said an investigation had shown all three had their headsets on, and there was silence each side of the message - indicating there had been no conflicting messages which would have prevented them hearing it.
The wing commander said he had only been able to decipher the message on the tape after listening to it several times. It would have come to Prestwick from the two radio receiversabout 70 nautical miles from the crash scene - on the Isle of Tiree and in the Scottish borders.
All three staff said they had not heard it - and the reason remained a mystery. There could have been some distraction from civilian air traffic controllers but staff had no recollection of this.
The wing commander agreed with John Mitchell QC, for the Crown, that the call remained a mystery. "Nothing on the equipment explains it?" asked Mr Mitchell. "No," said the wing commander.
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