Minnesota Attorney General Keith Ellison on Wednesday announced that Derek Chauvin, the former police officer who was arrested in the killing of George Floyd, is now charged with second-degree murder.
The charge has been increased from one of third-degree murder, which was laid last Friday.
Mr Chauvin knelt on Floyd's neck for more than eight minutes.
Three other former Minneapolis police officers – Tou Thao, Alexander Kueng and Thomas Lane, who were with Mr Chauvin at the time of the killing – were charged on Wednesday for aiding and abetting murder.
“George Floyd mattered. He was loved, his family was important and his life had value,” Mr Ellison said.
The Attorney General is a former congressman and the first Muslim American to hold such position.
"We will seek justice for him and for you and we will find it,” Mr Ellison said.
But he said that this could take time.
"We are moving as expeditiously, quickly and effectively as we can," he said. "But I need to protect this prosecution.
"I am not going to create a situation where somebody can say this was a rush to judgment."
An updated court complaint read that Mr Chauvin killed Mr Floyd "without intent to effect the death of any person, while committing or attempting to commit a felony offence".
Under US law, second-degree murder and aiding and abetting a murder could mean up to 40 years in prison.
The family’s lawyer, Benjamin Crump, called the announcement a “bittersweet moment”.
"We are deeply gratified that [Mr Ellison] took decisive action, arresting and charging all the officers involved in George Floyd's death and upgrading the charge against Derek Chauvin to felony second-degree murder,” Mr Crump tweeted.
Floyd's death on May 25 has sparked off protests across the US and around the world against systemic racism and police brutality against black men.
Protesters who have vented their anger in major US cities over the past week had demanded that the case be widened to include all the officers who were present during the incident.
"This is another important step for justice," said US Senator Amy Klobuchar, who is from Minnesota and is a potential running mate for Democratic nominee Joe Biden in the November 3 election.
Tens of thousands of people defied curfews and took to the streets of cities, coast to coast, for an eighth night in protest over Floyd's death and brutality against other black Americans.
Authorities took the unusual step of ordering curfews and bands of police in riot gear and other heavily armed officers patrolled, ringing landmarks and shouting at protesters while helicopters roared overhead.
The demonstrations have been mostly peaceful with less looting and vandalism overnight, and clashes between police and protesters were more sporadic.
President Donald Trump has said justice must be done in Floyd's case but also promoted a hard line on violent protests, threatening to use the military to end the rallies.