The FBI is looking at an "odd bit of buried foam" as possible evidence in the cold case investigation into criminal mastermind D.B. Cooper, according to private investigators.
The potential evidence was handed over to authorities last week by the team of sleuths who believe the foam made up a part of Cooper's parachute backpack, according to Daily Mail.
Cooper, one of the 20th century's most compelling masterminds, hijacked a Boeing 727 at Seattle-Tacoma airport in 1971 and held its crew and passengers hostage with a bomb.
Once his demand of US$200,000 (NZ$273,037) cash - the equivalent of US$1,213,226 (NZ$1,656,281) today - was reached and transferred onto the plane, Cooper had the crew take off before he parachuted out over the dense Pacific Northwest woods and disappeared.
The discovery of the foam comes just weeks after the FBI uncovered what is believed to be part of Cooper's parachute strap, which private investigators claim could lead authorities to his stolen fortune.
In addition, the FBI also received three "unknown" pieces of fabric that were found close to where the alleged parachute strap was located.
The team of private investigators is headed by TV and film executive Thomas Colbert and his wife Dawna. They established the dig site where the alleged items were found after receiving tips from what they claim are credible sources.
Colbert said earlier this month that the strap was found "right where a credible source claimed the chute and remaining money are buried."
He wouldn't publicly reveal the location to the media but said the potential evidence would be handed to the FBI for analysis.
Colbert said an unnamed couple led him to the site, which was corroborated by reports he had obtained from the FBI.
The amateur investigator sued the FBI for the documents after they announced they would no longer be investigating the case in July 2016.
He has spent several years conducting his own investigation into the mysterious crime, writing a book and producing a documentary series on it in the process.
It's widely believed that Cooper - whose real identity remains unknown and who was never seen again after the heist - died of exposure in the woods between Oregon and Washington.
He parachuted from the plane wearing just a suit in late November - hardly ideal conditions for surviving in the thick, chilly vegetation.
And until now, the only convincing evidence of his crime that has been found were three packets of US$20 (NZ$27.30) bills totaling $4,900 (NZ$6689).
They were found by a river on the states' borders in 1980, and confirmed to be from the hijacking.
"DB Cooper" is believed to have merely been a pseudonym used by the hijacker and not his real name.
Last September, Colbert announced that he believed Cooper's real identity was Vietnam veteran Robert Rackstraw, a highly decorated pilot who faked his death in 1978 to avoid a fraud trial.