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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.