The state Supreme Court on Tuesday sided with an online gun seller that argued it can't be held liable for a mass shooting committed by one of its customers.

The news comes as a major disappointment to gun control advocates, who had been buoyed by a prior Court of Appeals decision that a lawsuit against Armslist LLC could go forward, the first ruling of its kind in the nation.

But a 5-1 decision reversed that ruling, agreeing with Armslist and other advocates for internet sites as mere forum providers and that the website has immunity under the Communications Decency Act, and dismissed the suit.

The plaintiffs had argued that the Armslist site's functions and features are not neutral tools to assist in commerce but more a statement to criminals that "this is a safe place" for unlawful gun sales.

Armslist LLC is where a man obtained the gun used to kill three people and himself at a Brookfield spa in 2012.

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Zina Daniel Haughton and two coworkers were killed when her estranged husband, Radcliffe Haughton, shot up the Azana Spa where she worked. Four others were injured, and Radcliffe Haughton fatally shot himself.

Yasmeen Daniel, personally and as administrator of her mother's estate, sued Armslist.com, on which Radcliffe Haughton found someone to sell him a gun and ammunition while he was prohibited by a domestic violence injunction from having firearms.

A trial judge dismissed the case, citing the Communications Decency Act grant of immunity to "interactive computer services" that provide for communication among other parties.

But in a unanimous opinion, the Court of Appeals last year reversed that decision. It found Armslist might be liable, based on creating its own content that made it easy for prohibited people to buy guns, not for just being a conduit or publisher of others' content.

Armslist appealed and the Supreme Court heard oral arguments in February. Numerous outside groups, on both sides of the issue, filed friend-of-the-court briefs in the case.

Chief Justice Patience Roggensack wrote Tuesday's majority opinion. Justice Ann Bradley dissented. Justice Shirley Abrahamson had withdrawn from the case before the oral arguments and did not participate in the decision.

Contact Bruce Vielmetti at (414) 224-2187 or bvielmetti@jrn.com. Follow him on Twitter at @ProofHearsay.