Well, the person who could best be held liable, if anyone were to be, would be the shooter's mother, who owned the gun(s) in question, and failed to ensure they they were secured in such a way that her son, whom she knew or should have known to have serious mental health issues, would be unable to access them. (IIRC, she actually lockd them in a security container, but it was a cheap, key locking type, and he got access to the key by killing her.)
But 1) she's dead; and 2) her estate probably isn't worth enough to be worth suing, not after the lawyers get their cut, and any remaining money is split among all the victims' families.
The whole thing is funded by the Brady Bunch, et al., for the exact purpose already stated - to cost a gun company lots of $$$.