Any parent of an autistic kid will recognize this as their worst nightmare. Two Maryland teenage girls were charged this week with manipulating a teenage autistic boy to do unspeakable things. They allegedly asked him to masturbate and have sex with a family pet and then recorded it on their cellphones. They allegedly sometimes held a knife to his throat. They kicked him in the groin and pulled his hair. They got him to walk on a frozen lake knowing the ice was thin, and when he fell through a couple of times, they didn’t help him get out. His mom got home that night and asked why his clothes were wet, and he told her he’d fallen in the pond but also told her the girls didn’t force him to go in. And here’s the heartbreaker, from his mother, who was contacted by the Washington Post (the boy’s name is being withheld by the papers because he is only 16): “He doesn’t appear to be traumatized. He thinks these girls are his friends and is surprised the police are involved.”

The most baffling thing about this story is the reaction of the victim. The papers are calling the boy “autistic.” This is a broad term that can be used to describe a range of cases, from a child who doesn’t speak to Rain Man to Warren Buffett. But there are vast differences between the experience of autistic kids, and my guess is that the boy involved in the Maryland case falls into the category of the most socially vulnerable. Severely autistic kids are immediately recognizable as such. They may speak, but they can’t at all easily communicate, and their movements are often jerky and unpredictable. But the Maryland boy sounds more like he would be classified as having Asperger’s, before the American Psychiatric Association discontinued that as an official diagnosis. He went to a mainstream high school in Southern Maryland. He got good grades. He had a learner’s permit. And his mother trusted him enough to let him come home from school by himself since he was 12 years old.

But these sparks of intelligence, of competence and independence and perhaps “normalcy,” are precisely what make a kid like him so vulnerable. A kid with severe autism would be more protected, says Dan Griffin, a Maryland psychologist who treats autistic kids and says he has seen many bullying cases like this one, although milder. (Griffin has also written for Slate.) “The Aspie kids tend to be integrated into the population. They’re smart, so people don’t tend to judge them as vulnerable. They think, ‘This kid has an IQ of 140. He should know better.’ ” And although we live in an era that’s highly tolerant of neurodiversity and attuned to disability, “smart” disabled kids can fall through the cracks. Our parenting culture puts a premium on intelligence. A kid who can ace math tests generally provokes the jealousy of fellow parents, not their pity. So the fact that this boy is such a poor judge of people that he can mistake these two wicked girls as his friends is often not seen as a major life impediment.

The Maryland boy’s mother said: “I fear that people are going to blame me and say, ‘You have this mentally challenged kid; why don’t you know where he is all the time?’ ” I feel her pain. From the little we know of the mother, it sounds as if she had to work a full day. And many of the abuses happened during the snow days this year, when working parents sometimes don’t have any recourse but to leave their kids unattended. So she probably left for work in the morning, blinding herself to anything but the most hopeful version of the story: that her disabled son had enough life skills to make it through his day.

One of the girls, 17-year-old Lauren Bush, has been charged as an adult with several counts of assault and is being held in a Maryland detention center. The other girl is 15, so she is charged as a juvenile, and her name is being withheld. After the girls were arrested, the Maryland mother asked her son: “You’re going to stay away from these girls, right? You don’t plan on hanging out with them again, right?” His answer: “I don’t know.” After that conversation, she will likely never have peace of mind at work again.

Correction, March 14, 2014: This post originally misspelled Warren Buffett’s last name.