"I strongly believe that these developments are in the interest of justice for Mr. Floyd, his family, our community and our state," Minnesota Attorney General Keith Ellison said in announcing the charges.
The announcement came more than a week after Floyd was killed while in police custody in Minneapolis, sparking nationwide protests that call for the end to police violence against black citizens.
Derek Chauvin, who pressed his knee into Floyd's neck for nearly nine minutes, was previously charged with third-degree murder and second-degree manslaughter.
The new second-degree murder charge says he killed Floyd "without intent" in the course of committing assault in the third degree, according to an amended complaint.
Thomas Lane and J. Alexander Kueng, who helped restrain Floyd, and Tou Thao, who stood near the others, were not initially charged. Lane, 37, Kueng, 26, and Thao, 34, are now charged with aiding and abetting second-degree murder and aiding and abetting second-degree manslaughter.
Chauvin, 44, was arrested last week and is being held at the Minnesota Department of Corrections facility in Oak Park. His bail was increased to $1 million Wednesday, court documents show.
Lane, Kueng and Thao were taken into custody Wednesday and are being held on $1 million bail, county jail records show.
Kueng's attorney, Tom Plunkett, said in a statement that his client "was asked to turn himself in to face charges" Wednesday afternoon and was in custody 15 minutes later.
Second-degree murder and aiding and abetting second-degree murder are punishable by up to 40 years in prison. Manslaughter and aiding and abetting manslaughter are punishable by up to 10 years in prison.
Two autopsies on Floyd determined that he died by homicide. Minneapolis Police Chief Medaria Arradondo fired the four officers and said they were "complicit" in Floyd's death, and Floyd's family and protesters nationwide have called for them to be arrested and convicted for the killing.
According to the video and the criminal complaint, Chauvin kept his knee on Floyd's neck as Floyd pleaded that he could not breathe, as witnesses protested that he was dying, and even as Lane twice asked to turn him onto his side. Still, Chauvin kept his knee in position for almost three minutes after Floyd became unresponsive, the complaint states.