This story is a near repeat of the attack on the ATF agents in Mexico, which happened right after they had started trying to trace some F&F weapons they found at crime scenes. So, if it is the same MO, is it not unreasonable to wonder whether FP agents in the employ of the cartels were involved in that assassination as well?
They would have been able to find out where the ATF agents would be, so they could waylay them. Also, it might help to explain why the surviving ATF agent apparently unlocked the armored vehicle doors and/or rolled down the window, allowing the assassins access to the vehicle, if he knew one or more of the attackers.
Interesting synopsis of various reports
http://www.emptywheel.net/2012/08/29/did-the-cia-agents-in-mexico-shoot-at-their-pursuers/
“Also note, the same article says that when the uniformed police showed up one of the agents yelled out that they were diplomats traveling with a Navy guy, which is what stopped the shooting. Uh, during a gun battle? Yelled? And in what language, given that the Navy captain was purportedly their translator (even the one attached to the US embassy in Mexico)? Did he open the door to yell?”