Well, first I must say that these lawyers are swill. The reason they name such defendants in a lawsuit is: (1) hope for a settlement - some will pay nuisance value to make you go away, (2) cover their asses, cause if they sue other folks who may be liable, those folks will insist on dragging everyone who ever even *talked* to the snipers into the suit (spread the loss, of course) - note that # 2 probably isn't applicable here.
Now, I bet they're suing on a products theory, which would assert something to the effect:
1. The product was being used for its intended purpose;
2. it was foreseeable the product could be used in the fashion it was being used;
3. There was some defect in the product - either in its design or labelling/warning (this case, I would assume they would attack the design);
4. That there would be no deaths BUT FOR the gun; and
5. the gun was the actual cause of the deaths.
Also, they'll liikely sue for Negligence, asserting the gun maker:
1. Had some sort of a "duty" to the victims;
2. That it "breached" the duty in some fashion;
3. That its breach of the duty was the actual cause of the harm;
4. That the harm resulting from the breach was foreseeable; and
5. That there was harm done (i.e. deaths).
Just some FYI folks. You can see that they have a very hard case indeed trying to pigeonhole their notion into a legal theory.
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