After all, no one assumes that smugglers are bold enough to carry guns through an airport, on multiple occasions, rather than transport them north by car or truck. “Were they really stupid, or were they stupid-smart enough to figure out there was a loophole at the airport and use it to their advantage” and ensure speedy delivery? Ms. Todd asked.

The authorities had the suspect under surveillance for many months. Delta, aware of the investigation, notified law enforcement in New York whenever Mr. Henry was traveling on a Delta flight, Ms. Todd said. But detectives assumed that the suspect was flying between Atlanta and New York only after arranging the purchases in Georgia, while the guns were being driven north.

Obvious questions arise. Mr. Thompson described “a long-term investigation” by his office and the New York Police Department during which Mr. Henry took 17 flights “with guns on him” over seven months. So when did the authorities suspect that the guns were being transported on Delta flights? And to what extent were the Transportation Security Administration and Delta Air Lines aware of this?

“Once he left the airport, as part of our investigation, we started to follow him because we knew he was on that flight,” Mr. Thompson said at the news conference.

After the arrest was made this month, prosecutors in the district attorney’s office and New York police detectives laboriously reviewed hours of airport surveillance video in Atlanta and identified segments showing Mr. Henry carrying his backpack onto the Delta flight, Ms. Todd said. Then the penny dropped. “It was bugging them that these guns were getting onto the streets so fast. Then, lo and behold, it was like, ‘Oh my God, they’re carrying them on planes; they’re not driving them,’ ” she said.

“This office had absolutely no knowledge that guns were being transported by plane until the Dec. 10 arrest,” she added.

Representatives for Delta and the Atlanta airport declined to discuss the details of the investigation with me. A spokesman for the T.S.A., David Castelveter, confirmed that the agency “has been part of the investigation,” but declined to elaborate. “T.S.A. takes the potential for insider threats at airports very seriously,” and continually does threat assessments and airport criminal checks for all airline employees, Mr. Castelveter said.