President Joe Biden is urging protests in Tennessee to remain peaceful as officials plan to release video of an arrest that led to a motorist's death.
Five Memphis police officers have been fired and are facing murder charges after Tyre Nichols, 29, died three days after a traffic stop on 7 January.
Bodycam footage of the encounter is expected to be made public on Friday evening local time.
Lawyers for the Nichols family said it will show him being severely beaten.
"I'm sickened by what I saw," Tennessee Bureau of Investigation director David Rausch said on Thursday after reviewing the footage, describing the officers' actions as "absolutely appalling".
The city of Memphis is reported to be on edge and police have stepped up patrols there as they prepare for possible demonstrations.
Mr Nichols, a black man, was stopped by five police officers, who are also black, on his way home after taking photos of a sunset at a local park, an attorney for the family said.
Officials say he was suspected of reckless driving.
A first confrontation occurred as Mr Nichols attempted to flee on foot when officers approached his car, the local authorities said.
They said a second confrontation happened when officers tried to arrest him.
Mr Nichols later complained of shortness of breath and was taken to hospital, police said, where he was listed in a critical condition.
A lawyer for Mr Nichols' family said the bodycam footage showed Mr Nichols being pepper-sprayed, struck with a stun gun, restrained and kicked.
He likened the incident to the notorious footage of Los Angeles police officers beating black motorist Rodney King more than 30 years ago.
All five of the officers face charges of second-degree murder, aggravated assault, aggravated kidnapping, official misconduct and official oppression.
Tadarrius Bean, Demetrius Haley, Desmond Mills Jr, Emmitt Martin III and Justin Smith were booked into jail on Thursday. They all joined the Memphis Police Department in the last six years, and were fired last week.
'Failing of basic humanity'
President Biden released a statement on Thursday appealing for calm as authorities prepare to release the footage.
"I join Tyre's family in calling for peaceful protest," he said. "Outrage is understandable, but violence is never acceptable."
The city's police chief Cerelyn Davis, the first black woman in that role in Memphis, also called for calm amid what she said was a "failing of basic humanity toward another individual".
The Nichols family and their legal team privately reviewed the video footage of the arrest earlier this week.
"He was a human piñata," lawyer Antonio Romanucci said of its contents. "It was an unadulterated, unabashed, non-stop beating of this young boy for three minutes."
In a news conference on Thursday, lawyers for two of the ex-officers said their clients planned to fight the charges.
"No-one out there that night intended for Tyre Nichols to die," said a lawyer for one of the men.
Officials said Mr Nichols "succumbed to his injuries" on 10 January, but provided no further details. An official cause of death has not yet been disclosed.
His family say he will be remembered as a "good kid" who enjoyed photography and skateboarding.
The father-of-one, who worked at the parcel delivery company FedEx, had Crohn's disease and suffered severe weight loss, relatives say.
Reverend Al Sharpton, a US civil rights leader, told the BBC the alleged crime was particularly painful because of the officers' race.
"We fought to put blacks on the police force," he said. "For them to act in such a brutal way is more egregious than I can tell you."
"I do not believe these five black police officers would have done this had he been a young white man," he added.
California-based trial lawyer Adanté Pointer said instances of black men being killed by black officers rarely make the news.
"This case exemplifies that it is not simply a white versus black issue, but instead that this is a power dynamic that plays itself out no matter the race of the police officers," he told the BBC.
The FBI and the Department of Justice have opened a civil rights investigation into Mr Nichols' death.
The officers involved are members of a special team known as Scorpion - short for "Street Crimes Operation to Restore Peace in Our Neighborhoods".
The unit, which was created to police high-crime areas, is now under review, along with all of the city's specialised units, according to the city's police chief.