An East German colonel was a key informant in the 30-year hunt for the alleged Lockerbie bomb maker.
The FBI were able to identify the suspect from details given by the army officer who defected to the West.
He said a man he believed killed two US soldiers in a west Berlin nightclub in 1986 was a dark-skinned Libyan called Abu Agila Mas’ud.
The soldier said investigators discovered the name from hotel records where he had stayed.
For years the FBI were unable to identify the bomb maker nicknamed The Ghost.
US prosecutors are poised to file an extradition request to Tripoli where he is serving a 10-year term for bomb-making.
It would be the first US trial for the 1988 downing of Pan Am flight 103 over the Scottish town that killed 270 people.
A source said: “Prosecutors are confident they have identified the man they believe responsible for making the bomb.
“They have never given up on achieving the justice the families of those who died rightfully deserve.”
Abdelbaset al-Megrahi was jailed for life by a Scottish court in 2001 before dying in 2012.
A CIA double agent working at Libyan Airlines told investigators al-Megrahi had been with a dark-skinned Libyan.
And a bomb timer fragment found in the debris appeared identical to ones supplied to Libya by a Swiss manufacturer, who described a man who matched Mas’ud’s description.