On the afternoon before Thanksgiving in 1971, a man in a dark suit and a clip-on tie walked up to the Northwest Orient Airlines counter at Portland International Airport in Oregon. He bought a one-way ticket to Seattle, a thirty-minute flight. He paid cash. He gave his name as Dan Cooper.

He took seat 18C, at the back of the plane, ordered a bourbon and soda, and lit a cigarette. Shortly after take-off, he handed a note to a flight attendant.

She assumed he was giving her his phone number, and slipped it into her pocket unread. He leaned over and told her she'd better look at it.

The note said he had a bomb.

The quietest hijacking

He opened his briefcase just enough for her to see red cylinders and wires. Then, calmly, he dictated his demands: $200,000 in cash, four parachutes — two main and two reserve — and a fuel truck waiting in Seattle.

Most of the passengers never knew anything was wrong. The pilot announced a delay for a "minor mechanical difficulty", and the plane circled Seattle for about two hours while the FBI and the airline gathered the money and the parachutes.

Witnesses later described the hijacker as polite, even considerate. He asked for meals to be brought for the crew. He seemed to know a great deal about the aircraft — enough to know that a Boeing 727 had a set of stairs at the back that could be lowered in flight.

The jump

In Seattle, the passengers were exchanged for ten thousand twenty-dollar bills and four parachutes. The FBI had photographed the serial numbers of every note.

Then the plane took off again, with a skeleton crew, heading for Mexico City by way of a refuelling stop in Reno. Cooper gave precise instructions: fly at around 10,000 feet, slowly, with the landing gear down and the flaps partly extended. He sent the crew into the cockpit and closed the door.

At around 8:13 p.m., somewhere over the forests of south-western Washington, the crew felt a sudden change in pressure. The rear stairs had dropped. When the plane landed in Reno, the cabin was empty — apart from two parachutes he had left behind, and his tie.

He had jumped into the dark, into rain and wind, wearing a business suit and loafers, with a sack of cash.

The search

The FBI called the case NORJAK, for "Northwest hijacking". For years, soldiers and agents searched the area where he might have landed. They found nothing — no parachute, no body, no briefcase.

The name "D. B. Cooper" came from a mistake. Early in the investigation, police questioned a man named D. B. Cooper, who was quickly cleared. A reporter's error attached the initials to the hijacker, and they stuck.

What probably happened

Many investigators believe Cooper didn't survive. He jumped at night, in a storm, over rough, forested country, without proper gear. Some FBI agents have suggested he may not have been an experienced skydiver at all: one of the reserve parachutes he was given was a training dummy that had been sewn shut, and it was missing.

But without a body, no one can be sure.

The Cooper vane

The hijacking did change something permanent. After Cooper — and after several copycat hijackers tried the same trick — the authorities required a simple device on Boeing 727s: a small flap that stops the rear stairs from being lowered while the plane is flying. It's still known as the Cooper vane.

In 2016, after 45 years, the FBI announced it was ending its active investigation. The case, technically, remains open.