I initially heard that the BP was responding to a tribal police officers sighting of the hummvee, but that has not been mentioned in any of the latest reports.
When I was down there, the BP did not spend allot of time on the reservation. They responded to sensor hits and calls from the tribe, but did not do much random patrolling. Most of the patrols were in National Park area and highway 85.
Something's nagging at the back of my mind about the fact that the Mexicans were using a rez for this incursion. Surely they couldn't think that any of the tribes in the Southwest would fail to alert US authorities to incursions on their reservations. It's more likely that they would know whether or not tribal police were stretched thin, also knowing that BP wouldn't routinely work the rez because the tribe is officially a nation and BP would stay off unless called. BIA police should probably be helping tribal police on border reservations like this.
Southwest indian tribes have no fondness for the Mexican government that I know of, and border tribes have been badly treated by rurales as a matter of routine, I've heard. I've never heard of Tohono O'odham before. Are they Papago?