Minnesota Attorney General Keith Ellison will increase the charge against a fired Minneapolis police officer to second-degree murder in the death of an unarmed black man and level charges against the three other fired officers in a case that has led to more than a week of sometimes violent protests across the United States, the Star Tribune newspaper said on Wednesday.

George Floyd, 46, died after Derek Chauvin, a white policeman, knelt on his neck for nearly nine minutes on May 25, reigniting the explosive issue of police brutality against African Americans five months before the Nov. 3 presidential election.

Chauvin, 44, was fired and charged last week with third-degree murder and second-degree manslaughter. Second-degree murder - the new charge - can carry a sentence of up to 40 years, 15 years longer than the maximum sentence for third-degree murder.

The three others who were involved in the incident - Thomas Lane, J. Alexander Kueng and Tou Thao - will be charged with aiding and abetting murder, the Minnesota-based newspaper said, citing sources.

No one at Ellison's office could be reached for comment. Attorneys for the other three officers who may be charged did not immediately respond to requests for comment.

Protesters have demanded over the past week that they also be charged in the case as they have vented their anger in demonstrations that have at times led to widespread vandalism and looting in major cities.