The US Justice Department is opening a sweeping investigation into policing practices in Minneapolis after a former officer was convicted of the killing of George Floyd there, attorney general Merrick Garland has announced.

The announcement came a day after former officer Derek Chauvin was found guilty of murder and manslaughter over Mr Floyd’s death last May, setting off a wave of relief but also sadness across the country.

The black man’s death prompted months of mass protests against policing in the US.

The Justice Department is already investigating whether Chauvin and other officers involved in Mr Floyd’s death violated his civil rights.

“Yesterday’s verdict in the state criminal trial does not address potentially systemic policing issues in Minneapolis,” Mr Garland said.

The investigation is known as a “pattern or practice” — examining whether there is a pattern or practice of unconstitutional or unlawful policing — and will be a more sweeping probe of the entire police department and may result in major changes.

It will examine practices including use of force, and whether the department engages in discriminatory practices. It will also look into the department’s handling of misconduct allegations and its treatment of people with behavioural health issues and will assess the department’s systems of accountability, Mr Garland said.

It is unclear whether the years under investigation will begin when Mr Floyd died or before.

Merrick Garland
Merrick Garland (Andrew Harnik/AP)

Minneapolis Police Department is also being investigated by the Minnesota Department of Human Rights, which is looking into policies and practices over the last decade to see if it engaged in systemic discriminatory practices.

The 46-year-old was arrested on suspicion of passing a counterfeit 20 dollar bill for a pack of cigarettes at a corner market. He panicked, pleaded that he was claustrophobic and struggled with police when they tried to put him in a patrol car. They put him on the ground instead.

The centrepiece of the case was bystander video of Mr Floyd, handcuffed behind his back, gasping repeatedly, “I can’t breathe”, and onlookers shouting at Chauvin to stop as the officer pressed his knee on or close to Mr Floyd’s neck for what authorities say was about nine and half minutes, including several minutes after Mr Floyd’s breathing had stopped and he had no pulse.

Floyd’s death May 25 became a flashpoint in the national conversation about the deaths of black Americans at the hands of law enforcement and sparked worldwide protests.

At trial, Chauvin’s lawyer persistently suggested his knee was not on Floyd’s neck for as long as prosecutors argued, suggesting it was across his back, shoulder blades and arm.

President Joe Biden has promised his administration will not rest following the jury’s verdict in the case, saying much more needs to be done.

“‘I can’t breathe.’ Those were George Floyd’s last words,” Mr Biden said. “We can’t let those words die with him. We have to keep hearing those words. We must not turn away. We can’t turn away.”

The Justice Department had previously considered a pattern or practice investigation into the police department soon after Mr Floyd’s death, but then-attorney general Bill Barr was hesitant to do so at the time, fearing that it could cause further divisions in law enforcement amid widespread protests and civil unrest, sources told the Associated Press.

Mr Garland said the challenges “are deeply woven into our history”.

“They did not arise today or last year,” he added. “Building trust between community and law enforcement will take time and effort by all of us, but we undertake this task with determination and urgency knowing that change cannot wait.”