Radcliffe Haughton killed his wife, Zina Daniel Haughton, and two of her co-workers at a Brookfield spa before fatally shooting himself.
A restraining order prohibited Radcliffe Haughton from possessing a gun but he purchased one anyway from a person he met through gun trading website Armslist.com.
Daniel Haughton’s daughter filed a lawsuit in 2015 alleging the website lacked safeguards.
A Milwaukee judge dismissed the case, saying federal law provides legal immunity to websites that relay information provided by third parties.
The 1st District Court of Appeals disagreed, ruling Thursday that website operators aren’t immune from liability arising from their own conduct.