Three former Minneapolis police officers charged with violating the civil rights of George Floyd chose to do nothing as the 46-year-old Black man suffered a "slow and agonizing death," a prosecutor said Monday.

Tou Thao, J. Alexander Kueng and Thomas Lane are on trial for their roles in the May 2020 death of Floyd, whose fatal arrest was filmed by a bystander and sparked months of protests in the United States against racial injustice and policy brutality.

Former Minneapolis policeman Derek Chauvin, who kneeled on the neck of a handcuffed Floyd for nearly 10 minutes until he passed out and died, is serving 22 years in prison after being convicted of murder.

Thao, Kueng and Lane were the three other officers on the scene involved in the arrest of Floyd for allegedly using a fake $20 bill to buy a pack of cigarettes.

While Chauvin knelt on Floyd's neck, Kueng was on his back and Lane held his legs. Thao kept back bystanders who were telling Chauvin to get off the visibly distressed Floyd.

In opening arguments Monday, prosecutor Samantha Trepel said the defendants ignored their own training and pleas from bystanders who were concerned about Floyd's welfare.

"They watched as Mr. Floyd suffered a slow and agonizing death," Trepel said, urging the jury to hold the three men accountable "for choosing to do nothing and watch a man die."

"They chose to ignore what they saw and heard," she said.

The indictment charges Thao, Kueng and Lane with showing "deliberate indifference to (Floyd's) serious medical needs."

Thao and Kueng are accused of failing to intervene to stop Chauvin's "use of unreasonable force" against Floyd.

Lane does not face the second charge. Video of the arrest shows that on two occasions, Lane suggested Floyd be rolled over on his side.

'Custody is in your care'

Trepel said the law requires police officers to intervene when they see another officer using excessive force and to provide medical aid.

"In your custody is in your care," she said.

"(Floyd) said 'I can't breathe' not just once or twice but 25 times as he tried to alert officers he was dying," Trepel said.

Robert Paule, Thao's attorney, began his defense by saying what happened to Floyd was a "tragedy."

"However," he said, "a tragedy is not a crime."

A jury of 12 jurors and six alternates are hearing the case against the three officers.

Chauvin was an 18-year veteran of the Minneapolis police force; Thao had served for eight years while Kueng and Lane were new recruits, having joined the department in December 2019.

Thao, Kueng and Lane are to face Minnesota state charges in connection with Floyd's death in a trial which is now scheduled to begin on June 13 after repeated delays.

But in a sign of the importance of the case, federal prosecutors also charged the officers with violating Floyd's constitutional rights.

The federal trial is being held in a heavily guarded courtroom in Saint Paul, the sister city to Minneapolis.

All three men have pleaded not guilty.

The trial is expected to last at least two weeks. Unlike Chauvin's state trial, the federal trial is not being televised.