What did those Border Patrol Agents stumble into?

The FBI is investigating the incident on the Texas-Mexico border where one agent was killed and another seriously injured.

This thing smells.

Coyotes and smuggled persons know the deal.  They'd be sent back home.  I've even read that catch and release is back on because there isn't enough space to keep all the people they're apprehending.

So illegals would simply play by the rules and turn themselves over to the Border Patrol.

But these people assaulted the agents.

So what did they stumble into?  There is more to this story than we're being told.