One of the biggest U.S. online gun markets can be held liable for negligence for the private sale of a weapon used in a fatal shooting spree, Wisconsin’s highest court said in the first decision of its kind.

Armslist.com isn’t protected by the Communications Decency Act of 1996, which largely immunizes online service providers from responsibility for posts by third-parties, the state Supreme Court said in reversing a lower court’s ruling.

“The act provides immunity to website operators, such as Armslist, only when the allegations treat the website as the publisher or speaker of third-party content,” the court said Thursday in its unanimous ruling. “The act does not protect a website operator from liability that arises from its own conduct in facilitating user activity.”

The ruling is a setback to gun-rights activists and follows a number of deadly mass shootings, including a recent attack on a Florida high school that led to a curb on gun sales to younger buyers in that state. Armslist’s attorney, Joshua Maggard, didn’t immediately return a call.

The lawsuit was filed after a domestic-violence abuser used a weapon he purchased on the site to murder his estranged wife, Zina Haughton, and several co-workers in a 2012 attack on a spa in Brookfield. The ruling, which removed a key defense for Armslist, means the case filed by Haughton’s daughter will proceed to determine if the company is responsible.

Armsist’s website includes a notice at the bottom of the homepage telling users to “always comply with local, state, federal and international law” and says the company does “not become involved in transactions between parties.”

The shooter was prohibited from buying guns but evaded the background check through a private sale, the Brady Center to Prevent Gun Violence, which represented the plaintiff, said in a statement. Armslist allegedly entered the online gun business after Amazon.com and other sites barred gun sales.

“This court decision puts sites like Armslist on notice that they can and will be held accountable,” said Kris Brown, co-president of the Brady Center.