To Reuters, the first shots were indeed done by the LAF - but, somehow, they can report with certainty that they were "warning shots."
Warning shots that were aimed at the supervising officers!
Also, it is clear that UNIFIL troops were alongside the Lebanese forces. Why would they allow the LAF to shoot anything? Why wouldn't the LAF be embarrassed to fire even warning shots unless they knew that UNIFIL was going to cover for them?
Worse yet, UNIFIL knows very well that the IDF was on its side of the Blue Line. Israel informed them in advance of the action.