Fox News is reporting that two of the "straw men" buyers had felony convictions (not to mention outstanding warrants, an indictment, and other details, all of which would be barriers to buying firearms). Yet they managed to buy over three hundred firearms, and pass the background check. Arizona is very good about reporting prohibited persons to the Feds. The probability seems to be that someone "fixed" the background check system so these guys could keep buying arms and running them to criminals.
I'd assume that'd require a high-level "fixer." If this is correct, I think it's clearly aiding and abetting a felony, which makes those guilty liable for the felony itself. Passively observing a felony wouldn't cross that line. Coaxing reluctant dealers to make suspicious sales might cross it. But sabotaging the background check system so that illegal sales are approved is, I think, clearly over the line.
Hat tip to Howard Nemerov and PJ Tattler.