Mayor Bill de Blasio has railed against the so-called iron pipeline of these weapons, which have been tied to many crimes in the city and to the deaths of three of the four New York police officers killed in the last 10 months. The gun used in the killing of Officer Randolph Holder, 33, on Tuesday, has yet to be found.

The day after Officer Holder’s killing, Mr. de Blasio bemoaned the presence of so many illegal guns from out of state on the streets of New York. “Our officers do so much every day to protect us, and yet they grapple every day with an unrelenting flow of firearms into this city from outside,” he said, speaking alongside a mournful group of officers, including the police commissioner, William J. Bratton. “There’s a disconnect in our society that somehow people who say they believe in law and order still support the notion of the free flow of weapons, and so often it is our officers who pay the price.”

Image Donovan Bryant

The authorities say that Mr. Bryant, who was charged on Thursday with 51 counts of weapons sales, ran a prototypical operation: taking orders for guns from New York City — no questions asked — before obtaining weapons from South Carolina, North Carolina and Tennessee, and then returning to his native Brooklyn, where he kept an apartment. He would take one of two personal cars, a Chevrolet sport utility vehicle or a Hyundai, both retrofitted to allow him to drive, or the Greyhound bus, the authorities said.

“The majority of these buys would take place in the middle of the afternoon,” said Capt. Stephen Espinoza, the commanding officer of the Police Department’s Brooklyn North Squad who oversaw much of the police investigation. Mr. Bryant told the undercover officer that he felt comfortable using a wheelchair, Captain Espinoza said, “because he felt it made him less susceptible to attention to police.”