They conspired to violate federal firearms laws, I.e., the straw purchase for someone forbidden to possess firearms. Another crime.
Their biggest crimes?
Not keeping their fat mouths shut.
Being stupid enough to think that their blabbering wouldn’t get to the feds.
Lets face it, thanks to homegrown terrs, anything like this will be looked into with a microscope.
And remember that just having the precursors to certain items/things can get you in serious trouble.