Who planted the gun for Michael Corleone to use in the restaurant scene in the 1972 film "The Godfather"?

The person who planted the gun for Michael Corleone (Al Pacino) to use in the restaurant scene in the 1972 film "The Godfather" was a thug working for Peter Clemenza (Richard S. Castellano). Clemenza has Michael Corleone (Al Pacino) use a Smith & Wesson Model 36 for his restaurant meeting with Virgil "the Turk" Sollozzo (Al Lettieri) and Captain McClusky (Sterling Hayden). Michael Corleone uses the gun to get required revenge for his father's murder.

The gun is astutely hidden behind the tank of the old-fashioned toilet in the Italian restaurant in the Bronx, NY when Michael meets with Virgil Sollozzo (Al Lettieri) and Capt. McClusky (Sterling Hayden). During the tense meeting, Michael asks to be allowed to go to the bathroom, before which he is frisked for a second time. Once in the bathroom, after a frightening few seconds, he thinks the gun might not be there. Then he finally finds it taped behind the toilet tank; he removes it and tucks it into his waistband.

Michael returns and sits at the dinner table. He is tensing the broken jaw he got courtesy of McClusky. Sollozzo is talking. With the sound of a passing train reaching its zenith, Michael stands and shoots both men twice before following Clemenza's instructions to drop the gun as he walks out, not looking anyone else in the local restaurant in the eye.