It began as an operation targeting a cocaine ring in southern New Jersey. But after investigators listened to a wire tap, the case took on an unexpected focus: do-it-yourself firearms made from parts easily found online.

The suspects in New Jersey are accused of trying to sell several homemade AR-15 assault-style rifles that are known as ghost guns because they cannot be tracked. The AR-15 rifle is a powerful weapon similar to the one used to kill 11 people last year at a Pittsburgh synagogue.

On Monday, New Jersey law enforcement officials announced the arrests of four men who became the first defendants charged under a law passed last year that makes it illegal to buy, manufacture, possess or sell homemade guns in the state. California is the only other state that restricts such weapons, but New Jersey’s version is far more stringent, according to national gun control groups.

In fact, one of the suspects is heard on a wiretap specifically citing New Jersey’s law and saying they had to move the weapon sales out of state.

“Ghost guns are a real and dangerous menace,” Gurbir Grewal, attorney general of New Jersey, said in a news conference in Camden where he discussed the case. “They offer criminals, convicted felons, terrorists, domestic abusers, people who we all agree shouldn’t access firearms, the ability to access them.”

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New Jersey already has some of the country’s toughest gun laws, but buying parts individually and assembling them at home — often with the help of online videos and instructions — could allow people with criminal records and others who would not pass background checks to obtain guns.

Marketed to gun enthusiasts who enjoy building their own firearms, companies that sell kits are making it easier for guns to circulate illegally, law enforcement authorities and gun control advocates said.

“You go to some of the websites, they offer you discounts if you buy a pack of five or a pack of 10,” said J Adam Skaggs, chief counsel and policy director at the Giffords Law Center to Prevent Gun Violence. “Now, you ask yourself, am I going to buy a discounted 10-pack of ghost guns because I want to build myself 10 guns to use at the range? Or am I going to buy a 10-pack if I’m a would-be trafficker who wants to start a little factory of my own and sell these at a markup to people who can’t pass a background check?”

Across the country, there have been arrests involving ghost guns with the accused charged with violating other state and federal laws. In New York, a police sergeant was accused of selling ghost guns to members of a motorcycle gang who had criminal records.

The difficulty of disrupting illegal ghost gun operations is underscored by the circuitous route that led authorities to the trafficking ring in New Jersey.

The investigation began as a probe into a large cocaine ring in Lindenwold, a town in Camden County, Mr Grewal said. Law enforcement authorities only became aware of the illegal gun trafficking through conversations between two of the alleged members of the criminal network that were secretly recorded.

“They changed some law,” one suspect was recorded saying, “so we gotta go over to Philly to pick them up.”

A second suspect said, “We just gotta go out of state now.”

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The suspects would have parts for guns shipped across the Delaware River to Pennsylvania to get around the New Jersey law, and then would bring them back into the state, Mr Grewal said.

Law enforcement officials eventually recovered six AR-15 rifles made from parts without serial numbers, which were being offered on the black market for $1,100 to $1,300, nearly double the market price for an AR-15 assault-style rifle. They also intercepted parts for two more assault-style rifles that had been shipped to Pennsylvania.

Another eight men were also arrested as part of the illicit operation and were charged mostly with drug-related offenses.

Though these were the first arrests for trafficking ghost guns under the new law, Mr Grewal said other ghost guns have turned up in the state. State police had recovered “well over 15 in the last year,” he said.

“We are starting to see them at crime scenes,” he added. “We are starting to see them in our investigations. They are not an abstract threat. They are a real threat to public safety.”

There are multiple investigations into ghost gun manufacturers and criminal networks, Mr Grewal said. But, he added, responsibility also rests with manufacturers.

“When you see repeatedly the same address and the same individuals at the same location continuing to order parts and parts and parts and parts for the same types of guns, some red flags have to go up,” Mr Grewal said.