A Minnesota judge issued the verdict for Tou Thao, one of the Minneapolis policemen charged with manslaughter in George Floyd's death case, finding him guilty of aiding and assisting manslaughter on Monday night.
Mass protests started across the globe after a black man named George Floyd was killed by four policemen in the United States.
Thao's conviction marks the end of state and federal lawsuits against the four policemen.
Thao's conviction is "historic and the right outcome", said Minnesota's attorney general Keith Ellison.
He added: "While we have now reached the end of the prosecution of Floyd's murder, it is not behind us."
"There is much more than prosecutors, law-enforcement leaders, rank-and-file officers, elected officials, and community can do to bring about true justice in law enforcement," he said.
All four men have already been convicted on federal civil rights charges. Additionally:
One of the four policemen Derek Chauvin was convicted on state murder charges in April 2021. Another officer Thomas Lane was sentenced in September 2022 on a state charge of aiding and abetting second-degree manslaughter while a third cop J Alexander Kueng pleaded guilty to a state charge of aiding and abetting second-degree manslaughter in October 2022.
Mr Floyd, an unarmed black man, died in May 2020 when Chauvin pressed his knee into Mr Floyd's neck for more than nine minutes.
The two other officials held him down while Thao held back bystanders.
On Monday, Hennepin County Judge Peter Cahill said in his 177-page verdict that Thao "actively encouraged his three colleagues' dangerous prone restraint" of Mr Floyd.
"Like the bystanders, Thao could see Floyd's life slowly ebbing away as the restraint continued," Judge Cahill wrote in his verdict.
"Yet Thao made a conscious decision to actively participate in Floyd's death: he held back the concerned bystanders and even prevented an off-duty Minneapolis firefighter from rendering the medical aid Floyd so desperately needed."
Thao waived his right to a jury trial in the Minnesota case, opting instead for Judge Cahill to determine the verdict.
He also waived the right to testify and question witnesses.
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