I did not know. Was he trying to catch them?
No, he was their handler!
The FBI knew about the plan and allowed the muslimes to continue on with it.
They could have stopped them at any time but chose to allow them to arm up and drive to the venue. the FBI had no plans to stop them, apparently. For instance, they did not notify Garland PD.
That is a criminal conspiracy right there.
We would not have known any of this had an FBI agent not been caught fleeing the scene.