(Reuters) - New Jersey’s attorney general on Friday sued a California company, accusing it of conducting illegal “ghost gun” sales, in what he said was the first lawsuit by a U.S. state against a distributor of the firearms.

The civil lawsuit against U.S. Patriot Armory and its founder James Tromblee Jr was filed three days after Attorney General Gurbir Grewal announced the arrests of four men for conspiring to sell six homemade AR-15 assault rifles.

Such weapons are known as ghost guns because they cannot readily be traced.

Ghost guns include firearms assembled from kits or made with 3D printers, and are sold with parts to make them operational. Buyers do not need background checks, and the guns’ lack of serial numbers makes it harder for law enforcement officials to track them.

Apple Valley, California-based U.S. Patriot Armory did not immediately respond to requests for comment.

The four men were the first charged under a state law signed by New Jersey Democratic Governor Phil Murphy in November making it illegal to buy, manufacture, possess or sell ghost guns.

According to Friday’s complaint filed in Essex County superior court, U.S. Patriot Armory sold ghost guns, including kits and parts for AR-15 rifles, to New Jersey residents online without telling them the firearms were illegal.

The complaint also said the company even stated on its website: “Is it legal? YES!”

New Jersey is seeking a halt to further ghost gun sales in the state by U.S. Patriot Armory, a website disclaimer that such sales are illegal, and civil fines.

Grewal, also a Democrat, said he filed the lawsuit after U.S. Patriot Armory shipped an assault firearms kit to an undercover state investigator, who paid for it with a credit card online for $583.98.

The guns can fetch higher prices on the black market.

New Jersey is the second U.S. state with a law against ghost guns, after California. But the New Jersey law is more comprehensive, according to the Giffords Law Center to Prevent Gun Violence. (here)

Grewal said 15 ghost gun companies have already agreed not to sell the firearms in New Jersey, after he began sending letters last June demanding that they stop.

He urged the U.S. Congress and other states to adopt similarly tough laws, noting that criminals and gangs can buy ghost guns in other states and traffic them into New Jersey.