I don't know for sure, but I would think that the car could detect a collision. I also don't know for sure, but I would think that FSD would stop once a collision has been detected.
Wouldn’t this be protocoled by the event data recorder?
Fun fact, at least until very recently, if not even to this moment, AEB (emergency braking) is not a part of FSD.
If that were the case here, there wouldn't be a government probe, right? It would be a normal "multi car pileup with a fatality" and added to statistics.
With the strong incentive on the part of both the driver and Tesla to lie about this, there should strong regulations around event data recorders [1] for self driving systems, and huge penalties for violating those. A search across that site doesn't return a hit for the word "retention" but it's gotta be expressed in some way there.
1. https://www.ecfr.gov/current/title-49/subtitle-B/chapter-V/p...
Of course, that depends on the speed it was traveling at to begin with.