“...a manual search of the FBI’s indices...”
This suggests someone literally thumbing through hundreds - maybe thousands - of missing persons case by hand. Also, the FBI only involves itself in missing person cases if it is requested to by the local investigative agency or if it involves a kidnapping that crosses state lines.
The bottom line is that any suggestion that the FBI conducted a comprehensive search of all missing persons cases throughout North America is false.
I do recall some suspect files (I read hundreds of them) came from tips - some of the tips included the person being missing or hadn't been seen for some time and the like ... we arent in any position to really make a statement about this. I guess that's my main point. People's missing reports are sometimes hearsay. People sometimes go missing intentionally for reasons having nothing to do with DB Cooper! (like kids running from the Nazis during WWII and its aftermath ....).
If you had to choose between having the FBI's partial vs a missing person report - which would you take?
*According to NamUs (National Missing and Unidentified Persons System), more than 600,000 persons go missing in the United States every year as a conservative estimate. Anywhere between 89 - 92 percent of those missing people are recovered every year, either alive or deceased or surface on their own. A large fraction surface on their own.
I'm not knocking the effort of the FBI. I'm just pointing out that it doesn't appear to be a comphrensive cross-reference of missing person cases at that time. That may not have been possible in 1971.
Some have assumed that because Cooper wasn't reported as missing, then that means he survived the jumped and went on and lived a long life. My point is that is not an accurate assumption to make. People can be missing and not appear in any list or database or report. Cooper might have been among them.
Something else I did when I went through these databases was to look at middle-age John Does, i.e. unidentified bodies of middle-aged men found in Oregon and Washington state in the years surrounding the hijacking. The goal was to identify the *kind* of person who can disappear and/or die and not be missed.
These men fell into two categories: homeless substance abusers (one interesting one was a well-known drug abuser at a hobo camp), and what I labelled "mountain men", rural men without friends or family who often travelled deep into the forest to commit suicide late in their lives.
Neither archetype fit my profile for Cooper so I moved on, however it would be interesting to see more research in this area.