It seems the IRA has used a sniper team in the past in the same way we are seeing in the DC area today....LINK HERE!
The unit was known to operate from a "mobile platform"an-improvised armoured saloon car with a gun mount in the back -which made it almost undetectable.
After the attacks, which resulted in 11 deaths (including one in Fermanagh), in only 14 or 15 operations, the sniper was driven away leaving no shells or forensic contamination.
Someone seems to be using the same tactic today in the DC area..