According to the Telegraph, the Free Syrian Army shot down the helicopter searching the wreckage, using US-made missiles. “The Rebels’ usage of these American-made TOW missiles has increased over 800 per cent since Russia began air strikes against them at the end of September.”
Zerohedge quote sums it up:
“US-backed forces supported by Ankara and Riyadh are now using weapons supplied by Washington to shoot down Russian aircraft.”