In the developments of the Jessica Ridgeway case that now resides in the courts, one person emerged as a tragic hero: the mother who turned in her own son.

Not much is publicly known about Mindy Sigg or the reason she called police on her 17-year-old son Austin Sigg, who is charged in Jessica’s slaying.

The anguish behind that phone call Oct. 23 is something few can understand.

David Kaczynski can.

In 1996, Kaczynski was reading through his brother Ted’s writings and began to realize they had the same tone and content as the manifesto penned by the infamous Unabomber — the mysterious domestic terrorist whose bombs had killed three people and injured 23 others over a 17-year span.

David Kaczynski’s wife had earlier suspicions.

“My first reaction was, ‘She is wrong,’ ” he told The Denver Post in an interview last week from Woodstock, N.Y. “My brother had never been violent. I was in denial. It took a long time of perusing the manifesto.”

David Kaczynski eventually tipped off the FBI, which led to his brother’s arrest, conviction and life sentence. Ted Kaczynski, 70, is now incarcerated in federal prison in Florence.

David Kaczynski said he knew that if he didn’t act on his suspicions, more people could die. If he did act, his brother could land in prison for the rest of his life or be put to death.

“It was like seeing a nightmare that becomes real,” Kaczynski said. “What’s going to happen to my brother? What’s going to happen to Mom? What’s going to happen to our family? You ask yourself questions. Is there any horizon beyond this? Is there any life left? Or any happiness? Will we have to change our names or our identities to be related to such a serious offender?”

Kaczynski said he understands the anguish Sigg’s mother must feel.

“It would be really important for people to remember that this mother who turned in her son most likely is horrified by what he did but still loves him because she is his mother,” he said. “The first thing I would say to her is, ‘This is not your fault.’ ”

After his brother was convicted, David Kaczynski and his mother, Wanda, met with victims. A widow of one of the victims looked at Wanda and said, “You don’t deserve any of this. It’s not your fault. We don’t blame you.”

That brought enormous comfort, he said.

“My mother always knew on an intellectual level that it wasn’t her fault,” Kaczynski said. “She knew enough about mental illness that she knew this wasn’t the result of bad parenting. But on an emotional level, this is your child. You still feel this sense of responsibility.”

When authorities stormed Ted Kaczynski’s Montana cabin, they found a live bomb. David Kaczynski’s suspicions were finally confirmed. He knew that he had done the right thing.

“There was a realization that he would have hurt other people,” he said. “We probably saved someone’s life. You think that no one else has gone through something like this before. It’s only in the years since that we have connected with other families who have played a role in turning in a family member that we have understood that this isn’t a unique experience.”

David Kaczynski said he hasn’t heard from his brother since he has been in prison. He writes him but has never received a letter back. Their mother died last year.

He became the executive director of New Yorkers for Alternatives to the Death Penalty and started a program called Limits of Loyalty, which introduces young people to the notion of responsibility for countering violent activities.

“To stand by passively when someone is being hurt, bullied, or treated unfairly is to stand with the bully instead of with the victim,” Kaczynski wrote in a blog for the Albany Times Union.

But parents faced with troubling evidence about their own don’t always turn to law enforcement, said Jennifer Collins, a law professor at Wake Forest University and one of the authors of a 2007 paper, ” Privilege or Punish: Criminal Justice and the Challenge of Family Ties.“

The paper tells the story of a Maryland high school senior, Samuel Sheinbein, who was charged with murder after police found the burned and dismembered body of one of his acquaintances in an abandoned home.

Instead of facing trial, the 17-year-old boy was flown out of the country by his father, who later explained to The Washington Post: “I did some simple soul-searching, and I came to the conclusion that with all due respect to the law, I am first of all a father and only after that a citizen.”

Sheinbein’s mother said: “Any parent would go and would do what we are doing.”

The boy eventually pleaded guilty in an Israeli court and was sentenced to 24 years in prison.

Collins said she has followed the Ridgeway case and was impressed by the actions of Sigg’s mother.

“She protected the community,” she said. “She clearly saw a situation where her child could do tremendous harm in the community. And she did an incredibly brave thing.”

In a 1990 USA Today phone survey of 305 adults, 79 percent of respondents said they would turn in a family member. Eight percent said they would not.

But Collins said her research has also found many cases when families protect their own — even killing a family member who turns in another to police. And in 14 states — not including Colorado — family members cannot be prosecuted for harboring relatives who are fugitives.

Mindy Sigg did not respond to a request for an interview. Her son has been charged as an adult and faces 17 charges, including murder, kidnapping, attempted kidnapping and child sexual assault.

Her only published comment was to a reporter from The Associated Press who called her after the arrest became public.

“I made the phone call, and he turned himself in. That’s all I have to say,” said Mindy Sigg, before she broke down in tears and hung up.

Jeremy P. Meyer: 303-954-1367, jpmeyer@denverpost.com or twitter.com/jpmeyerdpost