Concerning the injection:
I am not a med tech, but I do read a lot.
Any material radioactive enough to kill quickly would burn the veins while being injected - this will definitely leave evidence an ME could find. It would also be excruciatingly painful for the victim.
Radiation kills by damaging the cells' ability to work - once the radioactive material was in the blood stream, the cells would start dying, leading to standard radiation poisoning, but maybe not as much radiation burn evidence on the skin.
One thing - the decomposition pattern of the body may not be normal - the normal gut bacteria would also be dead so although the body would break down, an astute ME might find differences from the 'norm'.
