Markov appears to be the only person to have been assassinated in a ricin attack in more than three decades. That's because to succeed, such an attack requires ricin to be directly injected, or in some other way be to directly ingested, by the victim. That makes ricin a poor choice for a terrorist trying to inflict mass casualties.
Further notes for Threepers. Even in a hypothetical 4th generation warfare conflict in an American context, ricin fails the test as a proper weapon for resistance forces.
A. When mailed in an envelope it is indiscriminate, thus cannot be guaranteed not to harm innocents other than the presumptive target.
B. Regime warmakers (the proper target for 4GW) do not open their own mail, thus they are hardly likely to look on such attacks as frightening. (Who among the Mandarin class cares if postal workers or secretaries are harmed?) The target for 4GW is the decision-making few inches of grey matter between the tyrants' ears. Even a fatality caused by ricin is at most a mere inconvenience and hardly likely to influence their behavior.
C. Because ricin delivered in this manner is both inefficient and indiscriminate it discredits the side that uses it.
D. Then there's the matter of timing, that pesky "No Fort Sumters" business. Such attacks now are also premature and thus morally repugnant and politically stupid.
E. Thus, while ricin letters are perfect weapons for a false flag attack by the regime, they fail the 4GW test a a proper weapon for insurgents.
What is a proper weapon? A precision-guided munition in the form of a rifle bullet targeted specifically at a known decision-maker -- after the regime initiates the war. That is a weapon that they both understand and fear. (See Diane Feinstein's obsession with .50 caliber rifles, for example.)