These observations led early investigators to focus on men with military parachute training, possibly with airline industry knowledge. Many of the most seriously investigated suspects have one or both of these characteristics.
What no one has been able to do is identify a specific individual who had these characteristics, matches the physical description, and can be placed on Flight 305 that evening by something more solid than circumstantial reasoning.
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## The Tie’s Microscopic Secrets
Among the physical evidence from the plane, the black clip-on tie has attracted the most sustained scientific attention. It is the item most directly and personally associated with Cooper: he wore it, he apparently touched it, and he left it behind.
The partial DNA profile developed from the tie in the mid-2000s created significant interest at the time. It was hoped that the profile might eventually be compared against a suspect’s DNA and produce a definitive match or exclusion. But the limitations of the profile prevented this from happening. The profile was partial and incomplete, and it could not support the kind of comparison needed for a legal identification.
The revelation in more recent FBI files that the tie contained unusual microscopic particles—including bismuth and unalloyed titanium—opened a different avenue of investigation. These materials are not commonly found together in ordinary environments. Some researchers who have analyzed the particle data have speculated that the tie’s owner may have worked in an industrial or technical environment where such materials were present, possibly in manufacturing or aerospace.