A mystery helicopter - possibly flown by the US Navy - could have shadowed the Chinook which crashed on the Mull of Kintyre in 1994, killing 25 intelligence officers.
The crash on June 2 made international headlines, with 10 top RUC officers killed at a key time in the peace process.
Retired MoD avionics expert David Hill has just published a book on the crash called ‘Their Greatest Disgrace’.
In it he notes that while the RAF Chinook ZD576 left Aldergrove at 17.42, two eye witnesses saw a second tandem rotor helicopter at Ballymena some 12 minutes before.
“What is immediately obvious yet not mentioned at any inquiry is the point of convergence [of both aircraft] - the Mull of Kintyre,” he says.
Radar records 30 minutes after ZD576’s take-off show what Mr Hill believes could potentially be the same mystery helicopter flying erratically away from the crash scene just after the collision.
A US aircraft could have been heading north to RAF Machrihanish just north of the Mull, where US forces were based at the time.
However the MoD said the object tracked may have been birds, atmospheric interference or flying wreckage.
No other Chinook was in Northern Ireland at the time, the MoD said, which leaves Mr Hill asking if the second aircraft might have been the almost identical US Navy helicopter, a CH-46 Sea Knight.
Alexander Bradley - a former soldier who had flown in Chinooks - saw the second aircraft at Ballymena some 12 minutes before ZD576 took off from Aldergrove.
The next sighting of the mystery chopper was by Hugh McCann just north east of Ballymena at Cargan.
Both men saw a tandem rotor helicopter like a Chinook but at the wrong time and place to have been ZD576.
ZD576 did not have radar and would have been unaware of the other aircraft. “No one seemed to want to discus the possibility that there was a second aircraft,” said Mr Hill. “Clearly this approach was by prior agreement.”
He added: “Clearly given the timings it is possible that both aircraft flew towards the Mull. It is possible the other aircraft loitered and then shadowed ZD576. Given the evidence that US Navy personnel were on scene shortly after impact, it is possible the proximity of the two aircraft led to confusion as to identity.”
US aircraft used high power emitters and the Chinook Mk 2 was susceptible to Electro Magnetic Interference; ZD576 suffered a communications breakdown just before collision, he added. The MoD declined to comment.