Prosecutors in Chicago on Thursday charged four black suspects with kidnapping and hate crimes over the assault of a mentally disabled white man shown live on Facebook, as the attackers hurled obscenities about whites and Donald Trump.
Video of the attack shows the victim, reportedly 18 years old, cowering in the corner of a room as four black assailants taunt and beat him, at times yelling "F.. Donald Trump" and "... white people."
President Barack Obama called the hours-long assault in his adopted hometown "despicable."
The two male and two female suspects - Jordan Hill, Tesfaye Cooper and Brittany Covington, all 18 years old; and Tanishia Covington, who is 25 - were due to appear in court on Friday.
The video, which quickly went viral, highlighted the rise of live video on social media platforms like Facebook and Twitter, and how it can sometimes be used to capture violent images that are then transmitted to all with no filter.
Hate crimes charges were filed because of the victim's "diminished mental capacity, the fact that they tied him up, the obvious racial quotes that they posted live on Facebook," Chicago police commander Kevin Duffin told a news conference.
Authorities did not know whether the victim - who knew one of his assailants from school - had voted in the November election or whether his political leanings had influenced the attack.
Instead, they detailed a series of events that began with what was supposed to be a visit between two school acquaintances over New Year's weekend, but which ended in a four- to five-hour-long assault on Tuesday, part of which was broadcast on Facebook.
"There was certainly no premeditation," Duffin said.
The disturbing 30-minute video quickly spread online before it was taken down by Facebook, prompting an outcry across the nation.
"What we have seen as surfacing, I think, are a lot of problems that have been there a long time," Obama told Chicago's CBS affiliate.
"Whether it's tensions between police and communities, hate crimes of the despicable sort that has just now recently surfaced on Facebook."
The US president also expressed optimism that subsequent generations can improve race relations.
"It doesn't mean that all racial problems have gone away. It means that we have the capacity to get better," Obama said.
In the video, two female and two male attackers are seen cutting off parts of the victim's clothes, hitting him and hacking off some of his hair at the scalp, causing him to bleed.
Authorities did not reveal the identity of the victim, who was tied up during the attack and had duct tape over his mouth. He was also forced to drink toilet water.
He was recovering with his parents after being released from the hospital, police said.
The unplanned attack came at the end of four days in which the victim and Hill, a school acquaintance, spent time together visiting friends.
Prior to being bound and assaulted, the victim "got into a play fight with Jordan and it escalated from there," Duffin said.
All four suspects face multiple felony charges, including for kidnapping, battery and hate crimes. The hate crime charge alone carries a sentence of at least a year in prison.
The perpetrators had expressed no remorse for their actions, police said.
Before police had announced charges, thousands of social media users, including far-right commentators, claimed without offering evidence that the assault was linked to the Black Lives Matter movement born in protest at police shootings of African Americans.
"Stand up with me and demand justice in Chicago for the beating of a disabled trump (sic) supporter by BLM," conservative US media personality Glenn Beck tweeted Wednesday night.
Many similar messages were posted under hashtags such as #BLMKidnapping.
Chicago-based civil rights leader Jesse Jackson issued a statement refuting any link to the movement.
"This uncivilized act of brutality broadcast on Facebook Live has nothing to do with our social or civil rights struggle. This is a sickness and is widely rejected," he said.
"No one affirms this insane and savage cruelty. It is a moral and spiritual collapse."