LOS ANGELES — A gun control group founded by former U.S. Rep. Gabby Giffords asked two web hosting companies on Friday to shut down websites selling parts and machines that help make untraceable homemade firearms known as “ghost guns.”

The Giffords Law Center to Prevent Gun Violence asked the providers that host GhostGunner.net and GhostGuns.com to disable the websites for violating the hosting companies’ terms of service.

The sites sell kits, components and machines that help create homemade semi-automatic weapons. It’s legal to build a gun in a home or a workshop, and advances in 3-D printing and milling have made it easier to do so. The kits can be purchased legally for a few hundred dollars without the kind of background check required for traditional gun purchases.

Attorneys for the gun control advocacy group said the homemade weapons are increasingly being used in crimes and asked each of the companies to “invoke its policies to help stem the tide of this illegal, deadly behavior.”

They argue that the hosting companies, Shopify and DreamHost, should invoke their ability to disable and terminate the websites. The group argues that the two sites sell “the sort of products that have already caused scores of senseless deaths — and are likely to cause many more, unless taken off the market.”

The gunman who killed his wife and four others in a rampage in Northern California earlier this month had been barred from having guns but built two semi-automatic rifles at home that he used in the shooting, authorities said. Federal officials are sounding the alarm about an increasing black market for homemade military-style semi-automatic rifles and handguns.

Cody Wilson, who runs GhostGunner.net, said the products he sells on his website are legal and in compliance with federal regulations. He has said although there is no legal requirement that he conduct background checks, he tries to take precautions to make sure the weapons aren’t used nefariously.

“This is an attempt to apply pressure to deplatform a legal, American business selling legal products to law-abiding customers,” he said.

Representatives for GhostGuns.com, Shopify and DreamHost did not immediately respond to emails seeking comment.