Appreciated @abihabib reaching out for this story. I think there are three big points here (thread): https://www.nytimes.com/2021/02/03/world/americas/mexico-arrests-police-officers-over-massacre.html
One is that violence against migrants is a systemic thing that results from both MX and US policies. Police at all levels are complicit or active participants in extortion, kidnapping, etc. Beyond structural violence, there is plenty of actual violence: https://www.presencia.mx/nota.aspx?id=160628&s=3
In part because of that, it shouldn’t be especially surprising that an ‘elite’ unit in Tamaulipas, one that had some degree of interaction with US law enforcement, was involved. More broadly, these militarized police forces just don’t do much to improve security for most people.
And that’s where it ultimately comes together: atrocities involving migrants aren’t as much about organized crime as they are about the state. Migrants are just one area where policies that make people vulnerable intersect with local dynamics of violence and corruption.