One question; three comments
1. How many "suicide/homicide by pilot" attempts have been initiated while there was another person (CC or FC) actually in the cockpit?
2. As mentioned, in the case of the JetBlue pilot with a mental aberration, the locked cockpit door facilitated protecting the flight - the opposite (perhaps) of the current tragedy.
3. In the EgyptAir crash, the CVR recorded a physical fight between the pilots for control of the aircraft, right up to the moment of disintegration. That case occured before post-9/11 security measures. Which cuts both ways - the attempt to overpower the errant pilot was unsuccessful, but at least someone was able to try.
4. As to the possible addition of the American FBI to the investigation - they happen to be one of the best criminal investigation units in the world, and especially adept at "psychological profiling" of mass or serial killers.
They may be able to spot something (or a pattern of somethings) in the history of the FO that would be missed by someone less expert (even someone sitting elbow-to-elbow with this pilot on the flight deck). They may be able to provide the "why" - if this does turn out to be an intentional act. Which may help the families cope in some way. And provide pointers or warnings that prevent a recurrence with a different pilot.
Just food for thought....