A jury is to hear closing arguments on Monday in the trial of the white ex-police officer accused of murdering African-American George Floyd, a case that laid bare racial wounds in the United States and has come to be seen as a pivotal test of police accountability.

Derek Chauvin, a 19-year veteran of the Minneapolis Police Department, faces a maximum of 40 years in prison if convicted of the most serious charge -- second-degree murder.

Chauvin was seen on video kneeling on Floyd's neck for more than nine minutes as the 46-year-old Black man lay handcuffed facedown in the street complaining he "can't breathe."

The harrowing video, which was shown repeatedly to the jury during Chauvin's three-week trial, sparked protests against racial injustice and police brutality around the world.

Eric Nelson, Chauvin's attorney, said at the opening of the trial that there was "no political or social cause" in the courtroom, but it has coincided with rising tensions from two other high-profile police killings.

Daunte Wright, a 20-year-old Black man, was shot dead in a Minneapolis suburb on April 11 by a white policewoman who apparently mistook her gun for her Taser, and a 13-year-old boy was killed by police in Chicago.

Wright's killing triggered several nights of protests in Minneapolis, and ahead of a verdict in Chauvin's case National Guard troops have been deployed in the Minnesota city where shop windows have been boarded up as a precaution.

With tensions high as a possible verdict nears, two guard members were slightly injured after at least one person opened fire from a car on a team of troops and police early on Sunday in Minneapolis, authorities said.

"The outcome that we pray for in Derek Chauvin is for him to be held criminally liable for killing George Floyd," said Ben Crump, an attorney for the Floyd and Wright families.

"Killing unarmed Black people is unacceptable," Crump told ABC News on Sunday. "We have to send that message to the police.

"Hold police officers accountable."

'It wasn't right'

Among the 38 witnesses who testified for the prosecution were some of the bystanders who watched Floyd's May 25, 2020 arrest for allegedly using a counterfeit $20 bill to buy a pack of cigarettes.

Darnella Frazier, the teenager who took the video that went viral, said Floyd was "scared" and "begging for his life."

"It wasn't right. He was suffering," Frazier said.

Genevieve Hansen, 27, an off-duty firefighter, said Chauvin and other officers rebuffed her offers to provide medical attention to Floyd.

Donald Williams, 33, said he called 911 to report a "murder" after Floyd was taken away in an ambulance.

Chauvin attended every day of the trial, dressed in a suit and taking notes on a yellow legal pad.

He spoke only once -- and that was out of the presence of the jury -- when he invoked his Fifth Amendment right not to testify in his own defense.

David Schultz, a law professor at the University of Minnesota and Hamline University, said he was not surprised by Chauvin's decision.

"The chances of him helping himself probably weren't going to be too good," Schultz said.

"I could imagine a scenario where the prosecution would play that nine minute and 29 second tape and ask Derek Chauvin what he was thinking when George Floyd said he can't breathe."

Much of the evidence phase of the trial involved testimony from medical experts about Floyd's cause of death and whether Chauvin had engaged in reasonable or excessive use of force.

A retired forensic pathologist put on the stand by the defense said Floyd died of cardiac arrest brought on by heart disease and the illegal drugs fentanyl and methamphetamine.

Medical experts called by the prosecution said Floyd died from hypoxia, or a lack of oxygen, from Chauvin's knee on his neck and that drugs were not a factor.

'Strong legal authority'

The defense also called a retired police officer who said Chauvin's use of force against Floyd was "justified."

Police officers testifying for the prosecution -- including the city police chief -- said it was excessive and unnecessary.

Schultz, the law professor, said the state had presented a strong case.

"They had to overcome the barrier of being able to prosecute police officers," he said. "Police officers have strong legal authority to use force."

A conviction on any of the charges -- second-degree murder, third-degree murder or manslaughter -- will require the jury to return a unanimous verdict.

Shultz said the defense, which called only seven witnesses, "may be hoping to persuade just one juror to get a hung jury."

The racially diverse jury is made up of six white women, three Black men, three white men, two mixed race women and one Black woman.

Two members of the jury will be excused by Judge Peter Cahill after closing arguments and the other 12 will be sequestered for deliberations.

Three other former police officers -- Tou Thao, Thomas Lane, and J. Alexander Kueng -- also face charges in connection with Floyd's death.

They are to be tried separately later in the year.