The new reports seem to indicate that there were no American troops involved - the diplomatic vehicle had two American civilians in it, and they had a Mexican officer who was a member of the Mexican Marine corp along with them. Mexican Federal Police in technicals had been chasing suspected members of the cartels, and lost them - they ran across the American vehicle, and mistook it. They pulled up behind them, pointed their weapons, and ordered them to halt. The driver of the American vehicle tried to make a run for it. The police subsequently opened fire.
The truth came out before the cover story did. Ooops.