The state Supreme Court dismissed a lawsuit Tuesday alleging a firearms website that enabled a man to illegally purchase the pistol he used in a mass shooting at a suburban Milwaukee spa six years ago is liable in the killings, ruling that federal law grants the site operators immunity.

The court ruled 5-1 that the federal Communications Decency Act protects Armslist LLC, a firearms classifieds website. The act absolves website operators of any liability resulting from posting third-party content.

Radcliffe Haughton’s wife, Zina Daniel Haughton, had taken out a restraining order against him that prohibited him from possessing a firearm. But he bought a semi-automatic pistol and ammunition in October 2012 from a person he met through Armslist.com, according to court documents.

The next day he opened fire at Azana Salon & Spa in Brookfield, where his wife worked. He killed her, two of her co-workers and wounded four others before he took his own life.

According to court documents, Haughton used an Armslist.com function that allowed him to bypass ads from licensed dealers, enabling him to avoid a background check.

The lawsuit filed in 2015 alleged Armslist’s operators should have known that the design of the site enabled illegal gun purchases. But Chief Justice Pat Roggensack, writing for the majority, said Tuesday that if a website’s features can be used lawfully the act immunizes the operators from liability when third parties use the sites unlawfully. Therefore all that’s left is to consider the site a publisher, triggering immunity under the act, she said.

“Regardless of Armslist’s knowledge or intent, the relevant question is whether (the) claim necessarily requires Armslist to be treated as the publisher or speaker of third-party content,” Roggensack wrote. “Because it does, the negligence claim must be dismissed.”

Justice Ann Walsh Bradley was the lone dissenter. She accused the majority of allowing Armslist to hide behind the federal law and called the decision a “manufactured interpretation” of the lawsuit’s arguments.