The April 2, 2012 edition of The New Yorker has a remarkable piece on Lyndon Johnson’s experience of Nov. 22, 1963. LBJ awoke an alienated vice president, dogged by scandal and likely in the twilight of his political career. You know how the story ends.

The piece is an excerpt from the upcoming fourth book in Robert Caro’s five-volume biography of Johnson. You might have thought every angle on Nov. 22 had been dulled, but this account is fresh and immediate.

The motorcade enters Dealey Plaza. From Johnson’s convertible, far behind the president, they hear a motorcycle backfire, or maybe a firecracker in the crowd. Then Rufus Youngblood, LBJ’s Secret Service agent, saw one of his colleagues in a car ahead “suddenly rising to his feet, with an automatic rifle in his hands.”

Whirling in his seat, Youngblood shouted — in a “voice I had never heard him ever use,” Lady Bird recalled — “Get down! Get down!” and, grabbing Johnson’s right shoulder, yanked him roughly down toward the floor in the center of the car, as he almost leaped over the front seat, and threw his body over the Vice-President, shouting again, “Get down! Get down!” By the time the next two sharp reports had cracked out — it was a matter of only eight seconds, but everyone knew what they were now — Lyndon Johnson was down on the floor of the back seat of the car. The loud, sharp sound, the hand suddenly grabbing his shoulder and pulling him down: now he was on the floor, his face on the floor, with the weight of a big man lying on top of him, pressing him down — Lyndon Johnson would never forget “his knees in my back and his elbows in my back.”

The piece is also a master class in transparent journalism. Caro carefully differentiates what is known, what is unknown, and what falls between. He treats certain sources with skepticism (“Carpenter, like Valenti, was an idolater …”) and is clear when he is speculating (“What was going through Lyndon Johnson’s mind as he stood there history will never know”).

Despite the apparent rigor, Caro glides from attribution to attribution, stitching a remarkably complete story. Jackie, her husband’s blood caked on her leg and glove, agreeing to appear with LBJ at his swearing-in on Air Force One:

Only once did Jackie’s voice change: when Lady Bird asked her if she wanted to change clothes. Not right then, Jackie said. “And then … if with a person that gentle, that dignified, you can say had an element of fierceness, she said, ‘I want them to see what they have done to Jack.’”