Companies selling so-called ghost guns, which are partially milled pieces of a firearm that can be used at home to assemble a fully operational weapon, were given a stern message Monday by New York Attorney General Letitia James: Stop selling in New York.

That's because, James said, those kits can be used to assemble an assault weapon, which are defined as such, and considered illegal, under New York state law.

"My office has reason to believe that you are engaged in the sale and advertisement to residents of New York of unfinished lower receivers and firearms components that are intended for the assembly of assault weapons," James wrote. "Assault weapons are illegal in New York, and the sale and/or advertisement of these products violates New York law."

James sent the cease and desist letter to 16 websites that she said manufacture firearms or firearm components, some of which also sell them online. Those websites, James said, often advertise their products as ways for individuals to avoid certain weapons bans.

One of the sites, James said, even advertised their product by saying, "if they don't know you have it, they can't take it."

The Attorney General's Office did not disclose which companies they sent the letter to because, they said, they can't risk advertising that the websites have sold unlawful products in New York.

The companies have been able to skirt federal regulations on gun sales by selling lower receivers as "80% complete," meaning that a user would have to mill it with tools to make it operational.

The lower receiver is the only part of a gun that's actually considered a firearm by law, and therefore subject to federal regulation. The other parts, independently, are not. That means buyers can use tools to complete the lower receiver and purchase other parts to assemble a so-called assault weapon.

Individuals who purchase an unfinished lower receiver online don't have to go through background checks. Buyers also don't have to hold a federal firearms license to purchase a "ghost gun," which is required for gun sales in New York.

They're called "ghost guns" because, once assembled, the firearms are typically unregistered and aren't branded with any serial numbers. That means members of law enforcement would likely not be able to trace it back to its owner, if it's found following a crime.

Not only can the products be used to make a weapon that's banned in the state, James said, the companies involved haven't warned buyers that assembling an assault weapon in New York is illegal and can carry various consequences, including a $5,000 penalty for each violation and potential criminal charges.

"Your website offers unfinished lower receivers that require simple milling in order to manufacture unregistered and unserialized assault weapons, despite the fact that such manufacture and possession are illegal in New York," James wrote. "Nor does your website adequately warn New York consumers that using these products in the manner for which they are intended and advertised could result in imprisonment and/or fines."

Assault weapons were codified into state law in 2013 when lawmakers passed the New York SAFE Act, which was approved in the wake of the mass shooting at Sandy Hook Elementary School and is widely considered one of the strongest gun control laws in the country.

The law defined assault weapons as semi-automatic rifles that can accept a detachable magazine and has at least one of several accessories considered to improve the firearm's accuracy or power, like a pistol grip or a collapsing stock.

James, in a statement with the letter, decried the use of firearms with military-style accessories as tools that help individuals carry out mass shootings.

"The proliferation of these types of weapons has not only caused indescribable suffering across the country, but gravely endanger every New Yorker," James said. "We must make sure that these illegal and untraceable guns are not built in New York."

One of the country's largest companies selling unfinished lower receivers, 80 Percent Arms, said when reached Monday by the New York Law Journal that it hadn't yet received notice from the Attorney General's Office to stop selling its products to residents in New York.

A manager reached at the company who identified himself as Chris but didn't give a surname claimed the company wasn't doing anything illegal by selling to people in New York.

"We always comply with all state and federal laws," he said. "All of the politicians may not like them, but to my knowledge they're still legal in the state of New York."

Democrats in the state Legislature, last week, introduced a bill that would explicitly outlaw those products in New York. The measure, sponsored by State Sen. Anna Kaplan, D-Nassau, would outlaw unfinished frames, or receivers, in New York and allow criminal charges for anyone who possesses or sells one.

That bill won't move until January, at the earliest, which is when the next legislative session is scheduled to begin in New York.

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