Four African-Americans accused of attacking an 18-year-old man with special needs while making anti-white racial taunts and broadcasting the assault on Facebook were charged with hate crimes in Illinois on Thursday.

Jordan Hill, Tesfaye Cooper, and sisters Brittany and Tanishia Covington were each charged with aggravated kidnapping, hate crime, aggravated unlawful restraint and aggravated battery with a deadly weapon. Tanisha Covington was the eldest at 24, while her sister and the two men were 18 years old.

"This should never happen," David Boyd, the victim's brother-in-law, said on Thursday. He said the family was overwhelmed by support expressed on social media.

The incident, part of which was streamed on the Facebook Live service on Tuesday, drew the attention of US President Barack Obama, who called it "terrible" in an interview with Chicago's ABC-TV affiliate.

"Part of what technology allows us to see now is the terrible toll that racism and discrimination and hate takes on families and communities," Obama said on Thursday.

The victim, who is white, has "mental health challenges", Chicago police said. He was not identified.

'Play fight' escalates

Police said the victim knew at least one of his alleged torturers, meeting Hill at a McDonald's restaurant in a northwestern suburb of Chicago late last week.

When he did not return home the next day, the victim's parents reported him missing. He was found by police days later, on Tuesday.

Police said Hill picked the victim up at the McDonald's in a stolen van. While the victim's parents reported him missing, their son and Hill spent the next two days together, visiting friends and sleeping in the van.

On Tuesday, a "play fight" between the two in the Covington sisters' apartment escalated, Chicago Police Commander Kevin Duffin said at the news conference.

The victim was tied up for four or five hours, gagged and beaten. His scalp was cut and he was forced to drink toilet water, Duffin said.

In the video, the attackers could be heard making comments about "white people" as the victim cowered in a corner, his mouth taped shut.

Police officers located the victim on Tuesday after neighbors complained about noise coming from the apartment. He was outside in freezing weather wearing only a tank top, shorts and sandals, police said.

He was taken to a hospital and later released. Members of the public alerted investigators to the Facebook Live video.

The four suspects were due to appear in a Chicago court on Friday.