"My eyes have witnessed things that my tongue has never uttered," he told Brooks. "Before I die, I want it written down."
Distracted and busy, Brooks never got around to finding out what it was until Johnson lay dying. She rushed to his bedside and heard him cry out, "Blood! Blood! Blood!"
Johnson, Brooks learned, had been a participant in the Mountain Meadows Massacre.
She had stumbled upon an astounding truth, Levi Peterson wrote in his 1988 biography of Brooks: "Within the collective heart of her homeland nestled a dark and damning guilt."
For the next few decades, Brooks combed the area for firsthand reports of the massacre - journals, letters and court proceedings. She found descendants of the participants accusing one another, trying to exonerate themselves. Particular animosity existed between the families of John D. Lee, the only person convicted of the murders, and Jacob Hamblin, who testified against Lee.
In 1950, Brooks published the first scholarly treatment of the massacre. It was, in part, her attempt to show that Lee did not act alone and that it was "rank injustice" to single him out.
"Inevitably she became more than an objective historian," Peterson wrote. "She became a minstrel of a gripping saga . . . a tragedian whose compassion pointed toward catharsis and forgiveness."
Though Brooks didn't say Brigham Young ordered the massacre, she did blame the Mormon prophet for inciting the populace, covering up the crime and allowing Lee to be the scapegoat. Not surprisingly, LDS Church leaders were unhappy with Brooks' account.
After her book was published, she and her husband, Will, no longer were welcome participants in the church. They were not asked to speak or lead. Eventually such shunning receded but her reputation as a maverick continued to the end of her days.
Since then, other writers and historians have found the massacre an irresistible topic. It has been the subject of a feature film, documentary, fictionalized accounts and serious historical research.
This month, the LDS Church published its first-ever account of the massacre in the Ensign, the church's official magazine. The piece was written by Richard E. Turley, managing director of the church's historical department and co-author of a forthcoming volume on the atrocity.
Though Peterson is pleased with the church's belated openness, he is troubled by what he views as Turley's attempt to place blame back on Lee.
"It suggests that Lee planted the idea for the massacre," Peterson said. "That article reverses Juanita's conclusions. If Turley has some evidence, I'd like to see it."
* Juanita Brooks, The Mountain Meadows Massacre, 1950
* Will Bagley, Blood of the Prophets: Brigham Young and the Massacre at Mountain Meadows, 2002
* Sally Denton, American Massacre: The Tragedy at Mountain Meadows, 2003
* "Burying the Past: Legacy of The Mountain Meadows Massacre," a 2004 documentary film by Brian Patrick
* "September Dawn," partly fictionalized film directed by Christopher Cain, released in August 2007