Four people have been charged with hate crimes in connection with a video broadcast live on Facebook that showed a mentally disabled man being beaten and taunted, threatened with a knife and forced to drink from a toilet.

The assault went on for hours, until the victim managed to escape.

The African-American suspects, who were jailed, can be heard in the video using profanities against their white victim, white people in general and President-elect Donald Trump.

CPD Arrested and Charged all four offenders with Hate Crime, as well as other charges. https://t.co/tqNH9eBISY — Chicago Police (@Chicago_Police) January 5, 2017

Police spokesman Anthony Guglielmi said investigators initially concluded that the 18-year-old man was singled out because he has "special needs", not because he was white.

But authorities later said the charges resulted from both the suspects' use of racial slurs and their references to the victim's disability.

It was also possible that the suspects were trying to extort something from the victim's family, police said.

The man's parents reported their son missing on Monday and told authorities they later received text messages from people who claimed to be holding him captive.

WHAT HAPPENED, ACCORDING TO CHICAGO POLICE: The incident began on December 31, when the victim and one of the suspects, 18-year-old Jordan Hill, met at a suburban McDonald's to begin what both the victim and his parents believed would be a sleepover. Instead, Hill drove the victim around in a stolen van for a couple of days, ending up at a home in Chicago, where two of the other suspects lived. The victim told police what began as playful fighting escalated, and he was bound, beaten and taunted with racial slurs and disparaging comments about his mental capacity. A downstairs neighbour who heard noises threatened to call police. When two of the suspects left and kicked down the neighbour's door, the victim escaped. A police officer later spotted the obviously disoriented man wandering down a street. The man was bloodied and wearing a tank top that was inside-out and backward. He had on jean shorts and sandals, despite freezing weather. Later, police discovered that a live video of the attack was broadcast on Facebook by the attackers. Video show the victim with his mouth taped shut and slumped in a corner of a room. At least two assailants are seen cutting off his sweatshirt, and others taunt him off camera. The video shows a wound on the top of the man's head. One person pushes the man's head with his or her foot. A red band also appears to be around the victim's hands. Authorities believe he was tied up for four to five hours.

The victim was a classmate of one of the attackers and initially went with that person voluntarily, police said.

"He's traumatised by the incident, and it's very tough to communicate with him at this point," police commander Kevin Duffin said.

The victim is a suburban Chicago resident described by Police Superintendent Eddie Johnson as having "mental health challenges".

Hate crime

"There was never a question whether or not this incident qualified as being investigated as a hate crime," Johnson said. But "we need to base the investigation on facts and not emotion."

The case heightened political tensions on social media, with some, including far-right activists, suggesting that it was linked to the Black Lives Matter movement. Police said there was no indication of any connection.

Most hate crimes are connected to the victim's race, but hate-crime charges can be sought in Illinois if a victim's mental disability sparked an attack, though it is rare.

In addition to hate crimes, the four were charged with kidnapping, aggravated battery and aggravated unlawful restraint. Three were also charged with burglary.

They were to appear in court on Friday.

Illinois law makes hate crimes "class 4 felonies," which generally call for a sentence of one to three years.

Kidnapping generally calls for three to seven years in prison. But attackers may be sentenced for aggravated kidnapping, which calls for a sentence of six to 30 years.

Chicago Cook County prosecutors identified the suspects as Brittany Covington and Tesfaye Cooper, both of Chicago, and Hill, of suburban Carpentersville. All are 18. A fourth suspect was identified as Covington's 24-year-old sister, Tanishia Covington, also of Chicago.

In Washington, White House press secretary Josh Earnest said the beating demonstrated "a level of depravity that is an outrage to a lot of Americans".

The video of the attack emerged at a time when police dealings with Chicago's black community are being closely watched. Less than a year ago, the nation's third-largest police force was sharply criticised by a task force for using excessive force and honouring a code of silence.

The department has also been the subject of a long civil-rights investigation by the justice department, which is expected to report its findings soon.