The Rev. Martin Luther King Jr. arrived at his Washington, D.C., hotel at 10 p.m. on Aug. 27, 1963 with work to do.

The next day he would climb the steps of the Lincoln Memorial and speak words that would reverberate through the rest of American history. But he still hadn’t written them down.

Organizers of the March on Washington proudly claimed that 100,000 protesters would be listening, but nobody knew how many would actually show up.

King knew only that it would be the largest group he had ever addressed and that it would be the speech of a lifetime. According to “King: A Life,” a new biography by Jonathan Eig, the civil rights icon spent about two hours that night writing an outline and 90 minutes polishing a final draft. He was done at about 3:30 a.m.

The completed speech was typed up, and copies were given to the press the next morning, ahead of King’s appearance.

On that day, Aug. 28, 1963, the 34-year-old Baptist preacher looked out over a crowd of perhaps 250,000, lining both sides of the reflecting pool, and began to read from his written words. Then just past the 10-minute mark something happened.

What happened is an indelible piece of American history, with a footnote that has persisted for 60 years.

Among those shouting “Amen!” and “Tell it!” in call and response to the preacher’s impassioned words, was gospel icon Mahalia Jackson, seated on the stage to King’s left. “Tell them about the dream, Martin!” she shouted.

Until now, many accounts of the speech mark that cry as the moment that King departed from his written speech and began to improvise, using themes from speeches he had given in Detroit in June and in North Carolina the previous year. Jackson, who had accompanied King at many of those appearances, was familiar with those themes.

Gospel singer Mahalia Jackson performed "I Been 'Buked and I Been Scorned" and "How I Got Over" before Martin Luther King Jr. gave his speech at the March on Washington in 1963. She also sang "Precious Lord" at King's funeral in 1968.

Credit: AP file

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Credit: AP file

Supposedly King, in that moment, left behind what was a conventional address, and launched into the sort of spirited sermon that moves congregations to shout and stamp.

Eig discovered that the story is a bit of a stretch.

The clue was in a recording created by Motown Records, long kept out of circulation. Motown chief Berry Gordy had planned to release an album of King’s speeches, and so Motown’s mobile engineers made an audio document of the events during the March on Washington. Their mobile gear generated a much higher fidelity recording than the radio and television sound files that most of us have heard, and picked up comments from many of those on stage.

The album was withdrawn after selling a small number of copies due to copyright disputes. After King was assassinated in 1968 the tape and masters were put in storage. A half-century later Motown made plans to create a “virtual reality” experience of the March on Washington, incorporating the recording.

Eig sought out a copy, and listened to it carefully. Mahalia Jackson was near the microphones, “and you can hear every word out of her mouth,” he said.

King had already launched into the practiced cadences of his “dream” motif when Jackson picks up the theme. King went on to borrow lines from Langston Hughes and the prophet Isaiah, and delivered a stem-winder.

In Eig’s masterful recounting of the speech, he makes that sequence of events clear. “King had already gone down that road before (Jackson) opens her mouth” he said.

Jackson certainly thrilled the crowd with her spine-tingling delivery of “How I Got Over” and “I Been ‘Buked and I Been Scorned.” But King was charting his own course when he began to speak of the dream, with words that still resonate today.

“I hoped,” said Eig recently, “I could help debunk that myth.”