The US Justice Department announced Wednesday an investigation into the Minneapolis police, a day after a white former officer was convicted of murdering George Floyd, signalling the Biden administration's intention to use federal powers to clean up systemic police abuse.

Tuesday's verdict raised cautious hopes in the Black community of a historic turning point in US justice, but the police killing of another African American cast a shadow over prospects for change less than 24 hours after Derek Chauvin was led from a Minneapolis courtroom in handcuffs.

Protestors outside the Minneapolis courthouse where former police officer Derek Chauvin is on trial
Protestors outside the Minneapolis courthouse where former police officer Derek Chauvin is on trial AFP / CHANDAN KHANNA

The ex-officer -- who knelt on Floyd's neck for more than nine minutes -- faces up to 40 years in prison after being found guilty of all charges over the death of the unarmed man.

The crime was recorded by a bystander whose video shocked the world, triggering mass protests across the United States and beyond, while also prompting a national reckoning on racial injustice and police brutality.

"But only with the passage of time will we know if the guilty verdict in the trial... is the start of something that will truly change America and the experience of Black Americans," Floyd's brother Philonise Floyd, the family's most outspoken member, wrote in a Washington Post opinion column.

A protestor outside the courthouse in Minneapolis
A protestor outside the courthouse in Minneapolis GETTY IMAGES / SCOTT OLSON

"It's up to all of us to build on this moment."

On Wednesday US Attorney General Merrick Garland announced a civil investigation to determine whether the Minneapolis Police Department systematically uses excessive force and "engages in a pattern or practice of unconstitutional or unlawful policing," including during legal protests.

It will also examine whether the city force showed a pattern of discrimination and unlawful treatment of people with behavioral health disabilities.

Protestors hold a banner outside the Minneapolis courthouse
Protestors hold a banner outside the Minneapolis courthouse AFP / CHANDAN KHANNA

At the Minneapolis intersection where George Floyd was killed, now a makeshift memorial, resident Helena Sere was "overwhelmed" in the aftermath of the guilty verdict but felt it stopped short of justice for Floyd.

Civil rights leader the Rev. Jesse Jackson speaks to demonstrators outside the courthouse where Chauvin is on trial
Civil rights leader the Rev. Jesse Jackson speaks to demonstrators outside the courthouse where Chauvin is on trial GETTY IMAGES / SCOTT OLSON

"Justice would be being able to bring him back," the 40-something African American told AFP. "But I would say the officer was made accountable for his actions, and I hope that's the beginning of change."

But with Sere and other Americans expressing relief that a rogue officer faced his due -- and as President Joe Biden said the conviction could mark "a giant step forward in the march towards justice in America" -- another instance emerged of police using lethal force.

Authorities in Ohio released body camera footage of an officer fatally shooting a 16-year-old Black girl, Ma'Khia Bryant, who appeared to be lunging with a knife at another girl.

Civil rights activists, including the Reverend Al Sharpton, pray with the family members of George Floyd during the closing statements of the Derek Chauvin trial. Former police officer Chauvin is being charged with the murder and manslaughter of Floyd, wh
Civil rights activists, including the Reverend Al Sharpton, pray with the family members of George Floyd during the closing statements of the Derek Chauvin trial. Former police officer Chauvin is being charged with the murder and manslaughter of Floyd, which sparked global protests against racial injustice and police brutality. AFPTV / Eleonore SENS

"As we breathed a collective sigh of relief today, a community in Columbus felt the sting of another police shooting," the Floyd family's lawyer Ben Crump tweeted Tuesday.

Police in Ohio's largest city urged against a rush to judgment in the case, in which an officer shot Bryant 11 seconds after exiting his car.

"Deadly force can be the response the officer gives" when they are confronted with someone employing such force against another person, Columbus Police Chief Michael Woods told reporters when asked whether the officer should have used other means to de-escalate the confrontation.

A protestor outside the courthouse in Minneapolis
A protestor outside the courthouse in Minneapolis GETTY IMAGES / SCOTT OLSON

White House press secretary Jen Psaki lamented the "tragic" shooting as she described a disturbing pattern in which "police violence disproportionately impacts Black and Latino people."

Another fatal police shooting of a Black man, Andrew Brown, happened Wednesday in the state of North Carolina during an investigation. The exact circumstances of the deadly incident were under investigation.

While the Chauvin trial progressed in Minneapolis, the Midwestern city was rocked by the fatal police shooting Sunday of yet another African American, 20-year-old Daunte Wright.

A memorial service was set for Thursday in Minneapolis, with civil rights leader Al Sharpton delivering a eulogy.

Last year's killing of Floyd as he lay face down and handcuffed saying repeatedly "I can't breathe" has prompted some police reforms, but advocates including Biden say more is needed.

Democrats in Congress demand what they say are long-overdue reforms, but they face opposition in the evenly split Senate.

The George Floyd Justice in Policing Act -- a sweeping package that bans choke holds, combats racial profiling and restricts officer immunity -- passed the House of Representatives with support from just one Republican.

Senate passage would require votes from 10 Republicans. None has signed on, although Biden says he will push for the bill.

Top Senate Democrat Chuck Schumer vowed to "not rest" until Congress passes key police reforms, and cautioned against celebrating Chauvin's conviction as a final justice.

"We should not mistake a guilty verdict in this case as evidence that the persistent problem of police misconduct has been solved, or that the divide between law enforcement and so many of the communities they serve has been bridged," Schumer said. "It has not."

George Floyd's brother Rodney said Black Americans have been victims of deadly injustice at the hands of authorities for centuries.

"We needed a victory in this case, it's very important, and we got it. And hey, we might actually breathe a little bit better now," he told AFP.

Three other ex-officers involved in Floyd's arrest go on trial later this year.