Also listed is Vintage Firearms, the store that sold the shooter the gun, and RMA Armament, the online retailer the shooter used to purchased body armor.
Assuming that Vintage Firearms and RMA Armament complied with the applicable ATF regulations, I'm not sure how they're responsible in any way, unless the point is to use lawsuits to bankrupt a legal business for acting in a legal way. It would make as much sense as suing Ford for manufacturing and selling a vehicle that was used to intentionally run over pedestrians. It's unreasonable to expect that a firearms retailer is going to be able to ascertain the future actions of every single person that purchases a firearm.