How did Krakauer become familiar with Chris McCandless's story and then write Into the Wild?
The best place to look for the answer to this question with textual evidence is in the "Author's Note" found before chapter 1 begins. This section begins with a brief paragraph that announces the body of a young man was discovered by moose hunters in the wilderness north of Mt. McKinley. Krakauer then tells readers that his editor from Outside magazine asked him to write a feature article on the mysterious person.
Shortly after the discovery of the corpse, I was asked by the editor of Outside magazine to report on the puzzling circumstances of the boy’s death.
The January 1993 article wound up being a 9,000 word article, but Krakauer admits that he couldn't shake the desire to find out more about McCandless, even after having written the lengthy article. Krakauer felt compelled to retrace McCandless's final year or so of life.
Unwilling to let McCandless go, I spent more than a year retracing the
convoluted path that led to his death in the Alaska taiga, chasing down details of his peregrinations with an interest that bordered on obsession. . . The result of this meandering inquiry is the book now before you.
Much of Krakauer's research involved interviewing people that McCandless had interactions with. The book begins with Jim Gallien's account of driving McCandless to the drop off point where he was never seen again. In addition to those interviews, Krakauer did a tremendous amount of research on Gene Rossellini, John Waterman, Carl McGunn, and Everett Ruess. Krakauer's goal was to show that other men lived (and died) in similar ways to McCandless.
Into the Wild, the book based on Christopher McCandless’ life was written by Jon Krakauer and published in 1996. In 1993 Jon had written an article about Christopher for the Outsider magazine. The immense response that Krakauer received for the article motivated him to delve deeper into the life of Chris and this resulted in the book.
Krakauer made all attempts to ensure the facts which form the basis of the book were accurate. These were collected by meeting hundreds of people which included family members, friends, colleagues and many others who had come across Chris while he was on his adventure that finally ended with his death in Alaska. Krakauer personally visited the locations where Chris had been to get familiar with the environment where he spent his days.
An important aspect that helped Krakauer connect to Chris’ life was his own ambition and several attempts made to spend time in the wilderness detached from others. The marked similarity of their lives is reflected in his words
I got away with it. Chris didn't. That's the only difference.