Chicago Police Commander Paul Bauer’s death was precipitated by a mundane offense. On February 13, 2018, a fellow officer spotted a man preparing to urinate on a wall in the city’s central Loop neighborhood. The suspect fled, and the officer radioed for assistance. Bauer responded to the call and encountered the man in a stairwell. The suspect shot Bauer six times, killing him.

Now, almost exactly two years after the murder, Bauer’s family is suing Armslist, the online marketplace that sold the Glock handgun used in the shooting. The lawsuit, filed in Wisconsin federal court on February 12, alleges that the website’s leaders — Brian Mancini, Jonathan Gibbon, and Broc Elmore, who are named as defendants — designed Armslist to “actively encourage, assist and profit from the illegal sale and purchase of firearms.”

“Our basic claim is that Armslist is negligent,” said Jonathan Lowy, the chief legal counsel at Brady, the gun violence prevention group representing Bauer’s family. “They entered a business that knew was rife with extreme risk to supplying guns. In entering that space, they did just the opposite of using reasonable precautions and designed a website in an extremely dangerous way that made it as easy as possible to buy illegal guns.”

The lawsuit cites a recent feature story published by The Trace and The Verge that examined unlicensed gun selling on Armslist. Our analysis of 2 million listings on the platform linked more than 700 unique phone numbers to high numbers of sales. The findings are suggestive of behavior that potentially violates federal law, which requires private sellers who cross the threshold of being “engaged in the business” of dealing firearms to obtain a federal license, and conduct background checks. The complaint argues that Armslist encourages high-volume private sellers to do business on the platform.

“The Armslist Defendants knew or should have known that if they had chosen to provide mechanisms by which lawful transactions are promoted and unlawful conduct is deterred, they could have prevented many or all of unlawful shootings involving weapons sold on Armslist.com,” the complaint states.

The Glock used by Bauer’s killer was sold by a Wisconsin man named Thomas Caldwell, who listed more than 200 guns for sale on Armslist in a two-and-a-half year period starting in December 2015. It was only after several of the guns sold by Caldwell turned up in crimes that federal authorities arrested him for unlicensed dealing. “The Armslist Defendants were also negligent per se because they are, at minimum, responsible as knowing accomplices or co-conspirators for Caldwell’s violations,” the lawsuit reads.

The suit is brought by Erin Bauer, Paul Bauer’s widow, and seeks yet-to-be-determined monetary and punitive damages. The plaintiff also asks the court to direct Armslist to institute policies to keep criminals from obtaining weapons.