Where's D.B Cooper?

On November 24, 1971, Thanksgiving, an unidentified man wearing a white shirt, narrow black tie, dark suit, raincoat, and sunglasses and carrying a briefcase arrived at  the Portland International Airport in Oregon. He identified himself as Dan Cooper and went on the Northwest Airlines Flight 305 , a Boeing 727 flight to Seattle that had 36 passengers. 
Quoted from The Guardian in 2007:
Once the plane was in the air, headed for Seattle, he lit a cigarette and ordered a bourbon and soda. Then he passed a note to the 23-year-old stewardess, Florence Schaffner, who at first assumed he was flirting, and didn’t bother to read it. “Miss, you’d better look at that note,” Cooper said. “I have a bomb.” She unfolded the piece of paper. “I have a bomb in my briefcase,” it read. “I will use it if necessary. I want you to sit beside me.” Schaffner sat down, and Cooper opened his bag, revealing a mass of batteries and wires.
After closing the bag, he demanded $200,000 in "negotiable American currency", four parachutes and a fuel truck in Seattle to refuel the aeroplane upon arrival. 
Schaffner relayed the instructions to the cockpit, when she returned Cooper had sunglasses on.
William Scott (the pilot), contacted Seattle-Tacoma Airport air traffic control, which informed the local authorities. The passengers were informed that their arrival would be delayed because of a "minor mechanical difficulty". The aircraft circled Puget Sound for around two hours to allow Seattle Police and the FBI to collect his demands. At 5:24 cooper was told that his demands were ready, and at 5:39 pm the aircraft landed. Cooper told Scott to land the plane on an isolated, brightly lit section of the tarmac and to turn off lights in the cabin to deter police snipers. Northwest Orient's Seattle operations manager, Al Lee delivered the items he had demanded. Once he had delivered everything, Cooper let all of the passengers including Schaffner to leave the aircraft. Then only Cooper, pilot Scott, flight attendant Mucklow, co-pilot Rataczak, and flight engineer H.E. Anderson were left on board. Cooper instructed them to fly south-east towards Mexico, but the co-pilot informed him that they had to refuel at least once to get that far. They agreed to refuel in Reno, Nevada. At about 7:40 pm they were off again. After takeoff Cooper told Mucklow to join everyone else in the cockpit and to remain there with the door closed. At about 8:00 the crew got a warning light flashing in the cockpit, indicating that the aft airstair apparatus had been activated. At about 10:15 pm the aircraft landed the FBI and police surrounded the plane as they were not yet sure that Cooper had left. But an armed search confirmed that he had gone. 
Picture of Boeing 727 with aft stair open:


What do you think happened to D.B Cooper?