It would be the difference between an investigative involvement and a military one.
Given the conflicting interests here we’ll likely never really know what happened to that plane.
But it would have been devlishly easy for grounds crew at the airport to put something on board. Or even a Russian convert to do so in their baggage. IIRC the lockerbie bomb was in a portable radio or walkman type device.
...I doubt the real story will be forthcoming as it's highly political.
Seems like we all agree on that part.