PORTLAND, Ore. (KOIN) – An amateur D.B. Cooper investigator plans to sue the FBI because he thinks he may have found a key to solving the case.

Eric Ulis has been investigating the case of D.B. Cooper, the man who hijacked a flight in the 1970s before leaping from the plane and disappearing. Ulis is now hoping to examine the hijacker’s tie because he believes investigators may have missed a feature that could contain DNA. 

Ulis tells Nexstar’s KOIN that he discovered a spindle in a tie “precisely like DB. Cooper’s,” that rises up. He says he talked to previous D.B. Cooper FBI investigators who told him they didn’t realize the tie has the adjustable spindle. 

KOIN spoke with Ulis, who is traveling to Washington D.C. to file a federal lawsuit under the Freedom of Information Act on Wednesday to try to force the FBI to allow him and a DNA expert to examine the tie, then use ancestry genealogy.

“That gives us the ability to take D.B. Cooper’s DNA and sort of reverse engineer this and identify his family, nephews, nieces, people of that nature,” Ulis said.

The mystery of Cooper’s true identity and what happened after he dropped out of the rear of a Boeing 727 on Nov. 24, 1971, the night before Thanksgiving, has become a decadeslong source of popular interest.

Cooper bought a $20 ticket from Portland, Oregon, to Seattle-Tacoma International Airport under the name “Dan Cooper,” but a mix-up in an early report about the “skyjacking” dubbed him D.B., and the name stuck, according to the Associated Press. Cooper, who wore dark sunglasses and a business suit, gave a note to a flight attendant just after takeoff that read, “Miss, I have a bomb and would like you to sit by me.”

His demands included four parachutes and $200,000 cash, which he received after the plane touched down at Sea-Tac to allow the rest of the passengers and a couple of flight attendants to get off. The plane took off, headed toward Reno, Nevada when he jumped somewhere over southwestern Washington.

While Cooper was never found, his parachute and some of the money tied to him were discovered later.

Ulis told KOIN the FBI recently denied his request to examine the tie under the Freedom of Information Act, which is forcing him to file the lawsuit.

“They’ve given access to the tie two separate times before to private scientists, private individuals, once in 2009 and once in 2011, and this could actually solve the case.”

In November, Ulis announced he had focused on a man named Vince Petersen as a D.B. Cooper suspect after tracing a rare chemical previously found on the tie to a lab where Peterson worked near Pittsburgh, Pennsylvania.

Solving the case is up to amateur sleuths since the FBI announced in 2016 it was no longer investigating the 1971 D.B. Cooper hijacking at Portland International Airport.

In a statement to KOIN, the FBI said it does not comment on potential litigation.