If I remember correctly, they knew he was modifying weapons, because their "informant" constantly asked him to saw off a shotgun until he finally relented and did.

Then they unannounced, showed up on his property while his son was out with his dog, first sign they were there was when they shot the dog, the son reacted by firing back and was killed.

The mother and baby incident was the hidden sniper going for a "twofer" is what the sniper supposedly told people. The mother had no idea there was even a sniper hidden in the forest somewhere, but the sniper claimed self-defense because the mother was holding a handgun.

That's the way I remember it