From The Accidental President: Harry S. Truman and the Four Months That Changed the World, by A. J. Baime (HMH Books, 2017), Kindle pp. 25-26:
In the Capitol, Truman headed through the long hallways, past the eight-foot statue of Benjamin Franklin, and down a marble staircase to Rayburn’s office, which was affectionately nicknamed “the Board of Education.” Truman arrived at about 5:05 p.m. There the Speaker of the House, “Mr. Sam,” was chatting with a couple of other guests. Here in this office, congressmen gathered to “strike a blow for liberty”—to drink whiskey. When asked why the room was called the Board of Education, Rayburn liked to say: “I guess some fellahs have been educated down there.”
Rayburn handed Truman his drink of choice—bourbon and water—then told him that a call had just come in for him, from Steve Early in the White House. Truman picked up the phone and dialed.
“This is the VP,” he said.
In a strained voice, Early ordered Truman to come to the White House “as quickly and quietly” as possible, and to use the main Pennsylvania Avenue entrance. Rayburn was watching Truman at this moment. “He is kind of a pale fellow . . . and he got a little paler,” Rayburn recalled.
Truman hung up. “Jesus Christ and General Jackson!” he said. He turned to Sam Rayburn. “Steve Early wants me at the White House immediately,” he said. He made for the door, and with his hand on the knob, he turned and said, “Boys, this is in this room. Something must have happened.”
The vice president walked out the door, then broke into a run. The Capitol hallways were nearly empty by this time, and Truman’s footsteps on the marble floor echoed through the corridors. He made it to his office in the Senate building quickly and out of breath. He grabbed his hat. “[I] told my office force that I’d been summoned to the White House and to say nothing about it,” he later wrote.
Outside it had begun to rain again. Truman found his chauffeur, Tom Harty, and off they went in the Mercury state car with no secret service detail. They arrived at the White House “in almost nothing flat,” Truman recalled, motoring through the Northwest Gate. Ushers greeted the vice president at the door, bowing and taking his hat. They led him upstairs via an elevator to the First Lady’s private study, where Truman found Mrs. Roosevelt, her daughter and son-in-law Anna and John Boettiger, and Stephen Early, sitting quietly. The First Lady approached Truman and put her arm around his shoulder.
“Harry,” she said, “the president is dead.”
Four words raced through Truman’s mind: The lightning has struck! “I was fighting off tears,” he later recalled. “It was the only time in my life I think that I ever felt like I’d had a real shock. I had hurried to the White House to see the president and when I arrived I found I was the president. No one in the history of our country ever had it happen to him just that way.”
He gathered himself. “Is there anything I can do for you?” he asked the First Lady.
“Is there anything we can do for you,” Eleanor Roosevelt answered. “For you are the one in trouble now.”