S5E5: Umbrella Assassin

On September 11, 1978, Bulgarian emigre, writer and broadcast journalist Georgi Markov died. Three days before, while waiting for a bus, Markov felt a sudden sting in the back of his thigh. Markov was soon hospitalized. After his death, an investigation ruled Markov had been poisoned. A pellet containing ricin had been shot into his leg. How had this political murder been executed?