Prince Felix Yusupov, Duma member Vladimir Purishkevich, and the Tsar's cousin, Grand Duke Dmitri Pavlovitch Romanov, important members of the St. Petersburg elite, finally took the lead in the decision to murder Rasputin.
There is some evidence that the British Secret Intelligence Service, worried that Rasputin may influence the Tsar to make peace with the German Empire and thus free up German troops for the Western Front, was also involved.
On the night of December 29/December 30, 1916 (16 December according to the Julian calendar that was still used in Russia at the time), Yusupov invited Rasputin to his palace on the pretext of his wife Irina needing his attentions as a healer. In a dining room in the palace basement, the two plied their guest with wine and cakes laced with potassium cyanide. The poison was ineffective, possibly because Rasputin was a heavy drinker and thus he suffered from achlorhydria (an absence of stomach acid, which is required to transform harmless potassium cyanide into lethal hydrogen cyanide), which meant that the poison had no effect on him. Alternatively, the sugars in the wine and cakes may have inhibited the cyanide, or the chemical used may have been non-toxic either deliberately or accidentally. A book by Edvard Radzinsky claims Yusupov may have deliberately fluffed the murder, because he was in love with Rasputin.
When the Siberian peasant failed to die, they shot Rasputin three times in the chest, back and head, and beat him around the head with a dumb-bell handle. They then tied the purported corpse into a sheet and dropped it through a hole in the ice into the river Neva, where the sturdy peasant finally drowned, having drifted under the ice, still fighting to free himself. Supposedly his penis was cut off and preserved after he died. The Russian museum that displays what they claim to be Rasputin's penis has measured its length at 30 centimeters, or about 12 inches.
