'Into the Wild' Chris McCandless' Sister Says He Was Determined to Cut Ties with Parents

— -- The sister of Chris McCandless, the hiker whose two-year odyssey across America and into the Alaskan wilderness was immortalized in the bestselling book "Into the Wild," says his expedition was not just about his love of nature and his adventurous spirit, but also reflected his intent to sever ties with his parents after what she calls traumatic childhood.

McCandless was 22 years old and newly graduated from college in May 1990 when he set out across the American West in a trip that would ultimately take him to the wilds of Alaska.

His journey was immortalized in the book, which is still on school curriculums across the country, and later turned into an award-winning movie. The book detailed how after graduation McCandless suddenly disappeared, not telling his family where he was going, giving his savings to a charity, using the name "Alexander Supertramp," and then hitchhiking across deserts and the Great Plains, riding the rails and living in a trailer park on the rugged California coast before finally making his way to Alaska.

Just over four months after he reached Alaska, McCandless' body was found by hunters in an abandoned bus he had found 30 miles away from the nearest town. McCandless had died of starvation. His corpse weighed just 67 pounds.

According to his journal, for 114 days, Chris lived in what he called his "magic bus." By the end, he had written that "death looms" and he was "too weak to walk out." He wrote that he had "literally become trapped in the wild."

McCandless has been criticized for being selfish and unprepared for his trip. He was there without a map and proper survival gear, and he had gone to great lengths to make it impossible for anyone to find him. But today, every spring, hikers from around the world still make the two-day trek to the bus, which has become a shrine to the young man many idolize as a symbol of adventure and a turning away from material things.

McCandless' sister, Carine McCandless, and his half-sisters, Shelly and Shawna, say there is something else that many people don't know. They are sharing what they say is a vital part of their brother's story, one that better explains why he had gone to such great lengths to vanish from their view on his epic excursion.

"Frankly I was asked every time I met with a group of people... why Chris left the way he did and why he felt the need to push himself to such extremes," Carine told ABC News. "I really watered down those answers for a long time ... and I really felt and learned that I was doing a disservice to Chris and all those people 'cause the greatest inspiration comes from truth."

In a new memoir, "The Wild Truth," Carine writes that she believes her brother's sudden disappearance and journey reflected his determination to separate himself from their parents and a traumatic childhood that she says they both shared.

"He wanted to really separate himself from a situation he felt was very toxic," Carine told ABC News.

The whole truth, she said, doesn't begin at the bus in Alaska, but rather at their childhood home 3,000 miles away in El Segundo, California.

"He was Chris. He was my protector," she said. "He was always strong. He succeeded at everything he tried."

Carine said she has fond memories of family vacations and time spent outdoors with her family. Her father Walt McCandless is a renowned rocket scientist who had worked for NASA, and her mother Billie McCandless built a consulting business with him.

But family life for Chris was much more complicated than it appeared to be. When Walt McCandless moved into a home with Billie and started having children with her, he was still married to his first wife Marcia and continuing to have children with her. In fact Walt was dividing his time between the two homes: one with Billie, Chris, and Carine and another with Marcia and their six children, including Chris' older half-sisters Shelly and Shawna.

"Both women were pregnant at the same time. I think a lot of people don't realize that," Shawna McCandless said. "This was a man that was in and out of our house. He would spend four or five days ... with us, and then be gone for a while. And then he'd come back."

Walt and Marcia finally divorced when Chris was 4 years old. In her new book, Carine says Chris did not learn until he was in high school that his father had still been married to Marcia when he was born, and that this realization upset him greatly.

After the divorce, Carine and Chris grew up knowing their half-siblings well, as Walt would take his children from his two marriages on vacations together and they would visit Walt and Billie's home, sometimes for extended periods.

But in her new book Carine also writes about what she says was a darker side of this already complex family.

She says -- and her half-sisters Shelly and Shawna agree -- that their father was controlling and domineering, with a hair-trigger temper that expressed itself in angry, verbal outbursts, threats, invective, and even physical attacks on his wives Billie and Marcia.

"There was a lot of choking and shoving," said Shelly McCandless, who lived in Walt and Billie's home with her half-siblings Chris and Carine during her senior year of high school.

"We would hear raised voices, and it would get louder and louder, and Chris would usually come and grab me and get me outside of the house," Carine said. "We would hear my mom say, 'Kids, kids, Come look what your father is doing to me,' and then he would scream right after, 'kids, get in here now, look what your mother is making me do.'"

"Dad would throw Mom down on the bed," Carine continued. "And he would be choking her, and she -- in between her breath -- she would be screaming out for help, and when he released her and walked out, she would run over to us and put her arms around us, and she would apologize to us."

Carine's half-sisters Shelly and Shawna said they also witnessed their father's volcanic temper in their home with their mother, Marcia.

ABC News obtained a copy of a restraining order Marcia obtained in 1972 against Walt -- shortly before their divorce -- in which she alleged that he had recently "struck" her in the arm and face, that he "has struck and threatened [her] on numerous occasions," and that she "feared for her safety."

In her book and in her interview with ABC News, Carine McCandless also describes an incident in which her father's aggression was directed at Chris, who was then in high school.

"Dad just hauled off and punched him right on the spin, and Chris just turned and looked to him and [had a] puff of disgust across his lips," she said. "And I saw this fear come across my father's face, and Chris just turned around and walked away."

Walt and Billie McCandless declined to speak to ABC News for this report. In a statement to ABC addressing Carine McCandless' new book, Walt and Billie McCandless said, "[This] fictionalized writing has absolutely nothing to do with our beloved son, Chris, his journey or his character. ... This whole unfortunate event in Chris' life 22 years ago is about Chris and his dreams." They call her allegations "hyped up" and "spiteful."

In her book, and in a new documentary that will air on PBS this month called "Return to the Wild, The Chris McCandless Story," Carine quotes from letters that Chris wrote to her before he left, letters she had kept secret for decades.

The letters contain numerous complaints about his parents and their childhood.

In one of the letters, Chris wrote to Carine that after his college graduation, "once the time is right, with one abrupt, swift action I'm going to completely knock them [Walt and Billie] out of my life. I'm going to divorce them as my parents. ... I'll be through with them once and for all forever."

"I'm not releasing the letters to hurt my parents," Carine McCandless told ABC News. "I'm releasing parts of his letters for people to get a better understanding of Chris because I think there's a lot of valuable lessons there, and I think it can help a lot of people."

Although Walt and Billie tried to locate Chris after he disappeared on his journey, even hiring an investigator, he made himself impossible to find, using a different name, going to great lengths to leave no traceable tracks.

"Walt and Billie deserve sympathy for losing their son, absolutely," Carine McCandless said. "I don't blame Walt and Billie for his death, but I do hold them accountable for his disappearance ... the fact that we didn't know where he was and he felt he had to be Alexander Supertramp."