A lawyer for the family of George Floyd, whose death sparked unrest across the US, has accused a police officer of "premeditated murder".
Minneapolis policeman Derek Chauvin has been charged with third-degree murder, but lawyer Benjamin Crump told CBS news it was a case of first-degree murder.
"We think that he had intent... almost nine minutes he kept his knee in a man's neck that was begging and pleading for breath," he said.
Looting is reported in Philadelphia.
Video from two Philadelphia TV stations on Sunday shows youths smashing several police cars and looting at least one store.
Reporting on the violence in West Philadelphia, the Philadelphia Inquirer says police cars were also set ablaze.
President Donald Trump tweeted: "Law & Order in Philadelphia, NOW! They are looting stores. Call in our great National Guard".
The Floyd case in Minneapolis has reignited US anger over police killings of black Americans.
It follows the high-profile cases of Michael Brown in Ferguson, Eric Garner in New York and others that have driven the Black Lives Matter movement.
In video footage, Mr Chauvin, 44, can be seen kneeling on Mr Floyd's neck for several minutes on Monday. Mr Floyd, 46, repeatedly says that he is unable to breathe.
"The fact that officer Chauvin kept his knee on his neck for almost three minutes after he was unconscious. We don't understand how that was not first degree murder. We don't understand how all these officers haven't been arrested," lawyer Crump said.
Three other officers present at the time have also since been sacked.
Derek Chauvin is due to appear in court in Minneapolis on Monday
For many the outrage over George Floyd's death also reflects years of frustration over socio-economic inequality and segregation, not least in Minneapolis itself.
There have been five nights of arson and looting in Minneapolis and the adjacent city of St Paul. Minnesota's Governor Tim Walz said on Saturday he was deploying the full Minnesota National Guard for the first time since World War Two.
Governor Walz said racism in his state had created the conditions for Mr Floyd's death.
The National Guard - the US reserve military force for domestic emergencies - said on Sunday that 5,000 of its personnel had been activated in 15 states and Washington DC. It added that "state and local law enforcement agencies remain responsible for security".
In the CBS interview, lawyer Benjamin Crump also said "we now have the audio from the police bodycam and we hear where one officer says 'he doesn't have a pulse, maybe we should turn him on his side', but yet officer Chauvin says 'no, we're going to keep him in this position'. That's intent.
"Also, the fact that officer Chauvin kept his knee on his neck for almost three minutes after he was unconscious."
The lawyer also said Mr Chauvin and Mr Floyd already knew each other before Mr Floyd's death in custody.
He said the Floyd family was "notified by the owner of a club that Derek Chauvin was an off-duty police officer where George Floyd was a security guard, and so they had to overlap".