Mara Salvatrucha is my bet!
I agree. The MS-13 gang (really a Latin American terrorist organization) is famous for attacking cops, often with little or no provocation.
That's reasonable. These guys are the type of thugs who would take on the police in Mexico. It wouldn't surprise me to see them take on the police in this country. The question still to be answered is why they picked this particular station. And after a few successful attacks, how many other criminal groups will be emboldened to go after police officers and their HQ's or sub-stations? Or individual officers who are on calls or having a lunch break. Or, off-duty officers just doing ordinary things like being at a movie, or mowing their lawns.