I have a hunch that the message either spoken or unspoken to the SS security detail was “If you see or hear anything suspicious ignore or look the other way, just let it happen” That’s how the system under most dictators work, as the subordinates know very well without being told what to do and what is expected of them.
For many of the security agents, it appeared to be their first day on the job. They probably did not question anything.
The Warren Commission has been recalled to duty.