Leon Czolgosz bought this small revolver—a short-barrelled, .32 caliber, Iver-Johnson—for $4.
He practiced concealing it, since he had a plan to hide it inside a handkerchief (also depicted in the photo) before using it on his target: William McKinley, the President of the United States.
On the 6th of September, 1901, Czolgosz made his way to the Temple of Music at the Pan-American Exposition in Buffalo, New York. There he would wait for the President's arrival.
At seven minutes past four, in the afternoon, McKinley reached out to shake the hand of his then-unknown assassin. Firing twice, Czolgosz would become a murderer within a week.
On Friday, the 13th of September, the First Lady - Ida McKinley - wrote the following words in her diary:
My precious left us this morning around 2 o'clock.
I hope the dear Lord will take me very soon.
Image, U.S. National Archives. PD
Quoted reference, from the PBS film America 1900, Part IV - "Anything Seemed Possible."