Did Nancy Lanza live in fear? Why many mothers of the mentally ill do.

Asra Q. Nomani, a former Wall Street Journal reporter, is the author of “Standing Alone: An American Woman’s Struggle for the Soul of Islam.”

When the doorbell rang one night in 2006, I opened my front door to find a diminutive figure standing before me, her face crestfallen.

“Mom, what happened?” I asked.

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“I thought I was going to die,” she whispered, my father gently guiding her inside.

My brother, then 43, had suddenly spun into a rage, she said. She rushed to her bedroom and locked the door; my brother broke it from its hinges, chasing her. She curled into a fetal position on her bed as he pummeled her back and head before walking away as quickly as he had sprung on her.

“Why did you let this happen?” I asked. But I knew the answer.

My mother, like countless other mothers on the front lines of America’s mental health battle, is in a risky position. She cares for a mentally ill child.

We don’t know whether Nancy Lanza ever thought her son Adam might hurt her — news reports say he had Asperger’s syndrome and a personality disorder and wasn’t known to be violent. But she’s part of an alarming number of parents who’ve been killed by their children.

Parricides, or the killing of parents, were 13 percent of U.S. family homicides in 2008, up from 9.7 percent in 1980, according to the FBI. About half of parricides involve children killing their mothers. The typical offenders, according to researchers, are adult sons who are ill and unemployed. Nearly half of them are 24 or younger, an age when, scientists say, the cognitive mind is still maturing; Adam Lanza was 20.

But what goes largely undocumented is the many mothers who live in fear of their children.

Kathleen Heide, a professor of criminology at the University of South Florida and the author of “Understanding Parricide: When Sons and Daughters Kill Parents,” says that most often, when people kill their mothers, they are sick and untreated.

As mental health services are cut, mothers become the agents-in-charge, serving as constant reminders to these children that they’re not well. “The moms are trying to keep their children safe. In illness, the sons many times resent their mothers,” Heide says, and that becomes particularly acute without treatment.

Even in popular culture, matricide is a theme for troubled youth. Rap star Eminem wrote a song, “Kill You,” lashing out at his mother for the dysfunction in his life that he attributed to her.

Too often, mothers search desperately for beds for their sick children, fighting with insurance companies and treatment teams. According to the nonprofit Treatment Advocacy Center, by 2010 the number of public psychiatric beds in the United States had dropped to levels last seen in the 1850s — about 14 per 100,000 people. Since 1955, the peak of psychiatric hospitalization, 95 percent of the nation’s public hospital beds for people with acute and chronic severe mental illness have been eliminated.

From Tucson to Aurora, Colo., young men suffering from mental illness have taken the lives of others.

“Those who kill are untreated most of the time. It’s not about access to weapons. . . . It’s about treatment,” says Dominique Bourget, a forensic psychiatrist at the University of Ottawa who has studied parricide. With the killing of mothers, she says, those who are sick often strike out at “the people most loving to them.”