CHICAGO — The appalling video seized the nation’s attention this week: A group of young people kidnapped, bound, beat, slashed, gagged, humiliated and threatened to kill a teenager with mental disabilities over nearly three days, and laughed about it as they carried out their acts.

But by far, the most unusual thing about the episode, advocates for people with disabilities have said, was not the abuse itself, but the fact that it was recorded. Violence against people with disabilities is far more common than most people realize, advocates have said, and frequently goes undetected or is not taken seriously.

The victim in the attack here, an 18-year-old white man, suffered from schizophrenia and attention deficit disorder, prosecutors said in court papers filed on Friday. The four accused attackers, ranging in age from 18 to 24, are black. The four appeared on Friday in Cook County Circuit Court and Judge Maria Kuriakos Ciesil ordered them held without bail.

The defendants are accused of multiple felonies, including hate crimes. Prosecutors said the victim, who was found by the police on Tuesday, was targeted based on his race and his disability. During a Facebook video made by one of the defendants, another can be heard denouncing white people, and, at one point, a man is heard saying he does not care if the victim is schizophrenic.