I'm going to preface this by saying that I am very open to the possibility that the US government is assassinating people that cause it trouble. No, I'm not open to it per se, rather I am sure it is happening.
The current US administration is already on record as acknowledging that they already kill US citizens without trial.
I am sure they are doing it in a more clandestine "ricin umbrella" way too, which to my knowledge they haven't admitted to. The sort of assassination that is being suggested here is the sort that I very suspect they are doing, but I cannot say that it is clear to me that it happened here.
The current US administration is already on record as acknowledging that they already kill US citizens without trial.