The hijacker left behind a cheap clip-on tie on the plane, which became a key piece of physical evidence.

Before jumping, Cooper took off his black tie. The FBI kept this $1.49 tie as one of their few physical clues. Years later, scientists managed to pull a DNA sample from the fabric. However, because the tie was handled by many people and the evidence is messy, it has not solved the case. It remains a frustrating clue in a file the FBI closed in 2016.