This incident was a retaliation for what was posted earlier (above), water rights and against the activist woman. The cartels don't care who, or how many, were involved. They only want their access to the water for their own profits. I suppose the police had held out long past their ability, or for inside political payoff, and this thing went down when the opportunity finally presented itself.
Don't believe that people will go without weapons because of religious belief. It will only be your cloak from the reality. Seventh Day Adventists eat meat in their own home, only to go vegetarian while at the church social. The same goes for Mormon folks when the need for weapons calls for hunting, protection, or criminal activity.
I doubt the Mexican gov't will allow much information to be published within their power. Maybe more information will come out, but I highly doubt the bulk of what REALLY happened will be known. I do know more of the story, but let's wait and see if anything more detailed will come about.