This combination of handout photos provided by the Hennepin County Jail created on June 03, 2020 shows (left to right) ex-officers Derek Chauvin, Tou Thao, J Alexander Kueng and Thomas Kiernan Lane booking photo. (HANDOUT / HENNEPIN COUNTY JAIL / AFP)

MINNEAPOLIS - Prosecutors on Wednesday leveled new criminal charges against four Minneapolis police officers implicated in the death of an unarmed black man who was pinned by his neck to the street during an arrest caught on video, sparking nine days of nationwide protest and civil turmoil.

George Floyd's death has become the latest flashpoint for long-simmering rage over police brutality against African Americans

Derek Chauvin, arrested Friday on charges of third-degree murder and manslaughter in the death of George Floyd, 46, was newly charged with an additional, more serious count of second-degree murder, according to court documents filed in the case.

The added charge, defined under Minnesota law as unintentionally causing another person's death in the commission of a felony offense, can carry a sentence of up to 40 years, 15 years longer than the maximum sentence for third-degree murder.

Chauvin, 44, was the white officer seen in widely circulated video footage kneeling on Floyd's neck for nearly nine minutes as Floyd gasped for air and repeatedly groaned, "Please, I can't breathe," before growing motionless while bystanders shouted at police to let him up.

Floyd, whom police suspected of trying to pass a counterfeit bill to pay for cigarettes, was pronounced dead at a hospital shortly after the fatal encounter on May 25.

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Three fellow officers dismissed from the Minneapolis police department along with Chauvin the following day were charged on Wednesday for the first time in the case - each with aiding and abetting second-degree murder and with aiding and abetting manslaughter.

Those three - Thomas Lane, J Alexander Kueng and Tou Thao - have also been taken into custody. Aiding and abetting second-degree murder carries the same maximum punishment as the underlying offense - 40 years in prison.

Floyd's death has become the latest flashpoint for long-simmering rage over police brutality against African Americans, propelling the highly charged issue of racial justice to the top of the political agenda five months before the US presidential election on Nov 3.

Scenes of protesters of all races flooding the streets - mostly peaceful but sometimes accompanied by arson, looting and clashes with police - have fueled a sense of crisis but also hopes of change.

The mass public activity followed weeks of lockdown due to the coronavirus pandemic, which forced millions of Americans out of work and disproportionately affected minorities.

Police arrest protesters who broke curfew by marching through Manhattan during a solidarity rally, June 3, 2020, in New York, calling for justice over the death of George Floyd. (WONG MAYE-E / AP)

Trial months away

Minnesota Attorney General Keith Ellison, a black former US congressman, has requested bail of US$1 million for each of the four former officers, court documents showed.

When reached by Reuters over the phone, Lane's attorney, Earl Gray, said he had not received any information on the charges yet. Attorneys for the other officers charged in the case did not immediately respond to requests for comment.

"This is a significant step forward on the road to justice, and we are gratified that this important action was brought before George Floyd's body was laid to rest," Benjamin Crump, attorney for the Floyd family, said in a statement.

At an afternoon news conference, Ellison said winning a conviction "will be hard," noting that Hennepin County Attorney Mike Freeman, whose office filed the original charges against Chauvin, is the only prosecutor in the state to have successfully convicted a police officer for murder.

Fully investigating the case "is going to take months," he said. Protesters had demanded the case be widened to include all the officers who were present during the incident.

Outside agitators

Several major cities scaled back or lifted curfews imposed for the past few days. But not all was calm.

In New York City's Brooklyn borough, police in riot gear charged into a crowd of about 1,000 protesters defying a local curfew, albeit peacefully, near an outdoor plaza, and clubbed demonstrators and journalists as they scurried for cover in a downpour of heavy rain.

The confrontation in Brooklyn seemed to be the biggest exception to a calmer night, hours after the new charges in Minneapolis.

Protests erupted in Minneapolis and elsewhere the night after Floyd's death and have since spread to dozens of cities large and small across the United States.

In many cities, demonstrators defying nighttime curfews have been met by police in riot gear firing tear gas, mace and rubber bullets to disperse unruly crowds. National Guard troops have been activated in several states to assist local law enforcement.

Authorities and some protest organizers have blamed much of the lawlessness on outside agitators and criminal elements taking advantage of the situation.

The demonstrations had grown mostly peaceful by Wednesday and clashes between police and protesters more sporadic.

Republican President Donald Trump has said justice must be done in Floyd's case but also touted a hard line against violent protests, threatening to use the military to restore order.

Defense Secretary Mark Esper said he did not back deploying troops to patrol the country.

Demonstrators leave a rally and head toward Seattle City Hall Wednesday, June 3, 2020, in Seattle, following protests over the death of George Floyd, a black man who died in police custody in Minneapolis. (ELAINE THOMPSON / AP)

"The option to use active duty forces in a law enforcement role should only be used as a matter of last resort and only in the most urgent and dire of situations. We are not in one of those situations now," he told a news briefing.

After long refusing to explicitly criticize a sitting president, former Defense Secretary Jim Mattis accused Trump on Wednesday of trying to divide America and roundly denounced a militarization of the US response to civil unrest. “Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try,” Mattis, who resigned as Trump’s defense secretary in 2018, wrote in a statement published by The Atlantic.

Protesters marched through downtown Washington again on Wednesday evening, chanting, "no justice, no peace," and "black lives matter." Another group gathered near the White House and Lafayette Square, where US Park Police drove peaceful demonstrators from the park on Monday before Trump walked through the park to a church to hold up a Bible for cameras.

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At the south Minneapolis street corner where Floyd was arrested, a crowd of hundreds stood in a vigil on Wednesday, some with their fists in the air, some weeping.

"These are baby steps," Kenneth Williams, 54, a black US Navy veteran who lives nearby, said of the newly announced criminal charges in the case. "Somebody should have stepped up and done something at the scene that day."

"Cops have been getting away with this for years, but now we have cameras," he added.