Diane Craglow was caring for a 14-year-old autistic boy named Connor Leibel in Buckeye, Ariz., one day in July. They took a walk to one of his favorite places, a park in an upscale community called Verrado. She was not hesitant to leave Connor alone for a few minutes while she booked a piano lesson for his sister nearby, because he usually feels safe and comfortable in places that are familiar to him, and he learns to be more independent that way.

When Ms. Craglow returned, she couldn’t believe what she saw: a police officer looming over the boy with his handcuffs at the ready, pinning him to the ground against a tree. Connor was screaming, and the police officer, David Grossman, seemed extremely agitated.

As Ms. Craglow tried to piece together what had happened, more officers arrived, spilling out of eight patrol cars in response to Officer Grossman’s frantic call for backup. Soon it became clear to Ms. Craglow that the policeman was unaware that Connor has autism, and had interpreted the boy’s rigid, unfamiliar movements — which included raising a piece of yarn to his nose to sniff it repeatedly — as a sign of drug intoxication.

As a graduate of Arizona’s Drug Evaluation and Classification program, Officer Grossman is certified as a “drug recognition expert.” But no one had trained him to recognize one of the classic signs of autism: the repetitive movements that autistic people rely on to manage their anxiety in stressful situations, known as self-stimulation or “stimming.” That’s what Connor was doing with the string when Officer Grossman noticed him while he was on patrol.