As Atzerodt and Paine fanned out to seek their targets, Booth, a celebrated actor, familiar to everybody who worked at Ford’s Theatre, had no trouble in slipping upstairs during the performance of Our American Cousin. Moving quietly down the aisle behind the dress circle, he stood for a few moments near the President’s box. A member of the audience, seeing him there, thought him “the handsomest man I had ever seen.” John Parker, the Metropolitan policeman assigned to protect the President, had left his post in the passageway, and the box was guarded only by Charles Forbes, a White House footman. When Booth showed Forbes his calling card, he was admitted to the presidential box. Barring the door behind him, so as not to be disturbed, he noiselessly moved behind Lincoln, who was leaning forward, with his chin in his right hand and his arm on the balustrade. At a distance of about two feet, the actor pointed his derringer at the back of the President’s head on the left side and pulled the trigger. It was about 10:13 P.M.
When Major Rathbone tried to seize the intruder, Booth lunged at him with his razor-sharp hunting knife, which had a 7¼-inch blade. “The Knife,” Clara Harris reported, “went from the elbow nearly to the shoulder, inside, — cutting an artery, nerves and veins — he bled so profusely as to make him very weak.” Shoving his victim aside, Booth placed his hands on the balustrade and vaulted toward the stage. It was an easy leap for the gymnastic actor, but the spur on his heel caught in the flags decorating the box and he fell heavily on one foot, breaking the bone just above the ankle. Waving his dagger, he shouted in a loud, melodramatic voice: “Sic semper tyrannis” (“Thus always to tyrants” — the motto of the state of Virginia). Some in the audience thought he added, “The South is avenged.” Quickly he limped across the stage, with what one witness called “a motion…like the hopping of a bull frog,” and made his escape through the rear of the theater.
Up to this point the audience was not sure what had happened. Perhaps most thought the whole disturbance was part of the play. But as the blue-white smoke from the pistol drifted out of the presidential box, Mary Lincoln gave a heart-rending shriek and screamed, “They have shot the President! They have shot the President!”
From David Herbert Donald’s outstanding biography of Lincoln.