State authorities say they’ve filed a first-of-its-kind lawsuit against a gun retailer accused of selling a kit to assemble an AR-15 rifle to an undercover investigator in New Jersey months after the Garden State banned “ghost guns."

A complaint filed in state Superior Court in Essex County on Friday accuses a California gun dealer, U.S. Patriot Armory, of ignoring a cease-and-desist letter sent by the state attorney general in December, weeks after Gov. Phil Murphy signed a new law making it a third-degree crime to purchase firearm parts with the purpose of assembling a gun without a serial number.

The lawsuit, filed against U.S. Patriot Armory and its owner, James Tromblee, Jr., comes just days after state authorities charged four men of illegally buying such gun kits from a separate company and selling unregistered AR-15s — the first arrests made under the new law.

State Attorney General Gurbir Grewal told reporters at a press conference on Friday that such “ghost gun” dealers exploit a “loophole” in state and federal firearms laws by selling “80 percent builds” — nearly complete firearms advertised with instructions on how to complete them.

Because the guns cannot be fired when they are 80 percent complete, authorities say, they can be sold online and shipped through the mail without background checks and other safeguards. Gun rights advocates argue this set-up is not a “loophole,” but simply a way for law-abiding gun owners to buy firearms more cheaply, or assemble them as a hobby.

After New Jersey banned “ghost gun” purchases in November, Grewal’s office sent cease-and-desist letters to at least 16 companies that sold such kits online. Many of them, he said, responded by informing customers on their websites that they could no longer ship to New Jersey.

That’s what happened in the case of the four men charged criminally with buying and selling “ghost guns” earlier this week, authorities claim. Two of them were allegedly captured on a tapped phone line discussing plans to have gun parts shipped to Pennsylvania to circumvent New Jersey’s ban.

But Grewal told reporters Friday that U.S. Patriot Armory ignored the state’s demand to stop shipping to the Garden State. Authorities are now asking for a court order to stop the company from selling “ghost gun” kits in New Jersey.

State officials say a gun retailer shipped an undercover investigator a "ghost gun" kit after Jersey banned them last year. Here's Consumer Affairs Director Paul Rodriguez alongside the AG: pic.twitter.com/uWHDWmUFHe — S.P. Sullivan (@spsullivan) March 22, 2019

He said an undercover investigator ordered “all the parts to make a fully operational AR-15-style rifle” for $583.98 in February, and that the company shipped them to a New Jersey address “without conducting a background check, without including a serial number, and without a warning that the gun we were buying was illegal in our state.”

Paul Rodriguez, the director of the attorney general’s Division of Consumer Affairs, told reporters the company made “misrepresentations” on its website, including statements such as “Is it legal? YES.”

Last week, Rodriguez said, an AR-15 kit arrived at their undercover address, “with all the components necessary to assemble this deadly and illegal weapon.” A State Police firearms expert later confirmed it could be assembled and fired, authorities claim.

This image, included in a court filing, shows the "kit" an undercover investigator allegedly purchased online allowing them to assemble an AR-15 rifle in violation of New Jersey law.

The lawsuit accuses the company of violating New Jersey’s Consumer Fraud Act by failing to inform customers many of their products are illegal in the Garden State.

A message sent by NJ Advance Media to U.S. Patriot Armory through its website seeking comment was not immediately returned.

As of Friday morning, the company’s order page appeared to allow customers to choose New Jersey as a shipment option.

The attorney general said some of the 15 other gun dealers who no longer ship to New Jersey advise their customers to change their delivery address, arguing gun dealers were not doing enough to make sure they weren’t running afoul of state law. Grewal declined to say whether his office was investigating other sellers.

The attorney general called on Congress to pass a federal ban on such weapon kits.

S.P. Sullivan may be reached at ssullivan@njadvancemedia.com. Follow him on Twitter.

Have a tip? Tell us. nj.com/tips

Get the latest updates right in your inbox. Subscribe to NJ.com’s newsletters.