"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.
Derek Chauvin, who had his knee into Floyd's neck for nearly nine minutes -- an incident recorded on cellphone video -- initially was charged with third-degree murder and second-degree manslaughter after Floyd died on May 25.
The new second-degree murder charge accuses Chauvin of killing Floyd "without intent" while assaulting him.
Authorities also charged former Officers Thomas Lane, 37, and J. Alexander Kueng, 26, who helped restrain Floyd, and Tou Thao, 34, who stood nearby with aiding and abetting second-degree murder and aiding and abetting second-degree manslaughter.
Separate autopsies identified homicide as the cause of Floyd's death.
Minneapolis Police Chief Medaria Arradondo fired Chauvin, as well as the others, whom he called complicit.The criminal complaint, supported by video evidence, accuses Chauvin of keeping his knee on Floyd's neck while Floyd told him he couldn't breathe and witnesses said he was dying.
Chauvin kept his knee there for nearly three minutes after Floyd lost consciousness, the complaint says.
Floyd family attorney Benjamin Crump called the announcement "a bittersweet moment."
"We are deeply gratified that (Ellison) took decisive action, arresting & charging ALL the officers involved in #GeorgeFloyd's death & upgrading the charge against Derek Chauvin to felony second-degree murder," Crump tweeted.
However, Crump said Chauvin should be charged with first-degree murder, which requires that prosecutors prove he intended to kill Floyd.
"The Attorney General has informed the family the investigation is ongoing, and if there is evidence to support first-degree murder conviction, they will charge it," the attorney added.
Gov. Tim Walz appointed Ellison to take over the case from Hennepin County Attorney Mike Freeman on Sunday.
Ellison insisted that the protests had nothing to do with the decisions announced Wednesday, which the former Democratic congressman said his office made "based on the facts that we gathered since this matter occurred and made these charges based on the law that we think applies."
He cautioned, however: "We're confident in what we're doing, but history does show that there are clear challenges here."
Walz welcome the charges, which he called "a meaningful step toward justice for George Floyd.
"But we must also recognize that the anguish driving protests around the world is about more than one tragic incident," the governor said.
"George Floyd's death is the symptom of a disease. We will not wake up one day and have the disease of systemic racism cured for us. This is on each of us to solve together, and we have hard work ahead. We owe that much to George Floyd, and we owe that much to each other."
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