Transcript for Great uncle of Laquan McDonald on Chauvin trial: ‘American justice is on trial again’
Once Mr. Floyd had stopped resisting and certainly once he was in distress and tried to verbalize that, that should have stopped. Reporter: A compelling condemnation of Derek chauvin from none other than his former boss, Minneapolis police chief medaria arradondo as his trial for the death of George Floyd entered its second week. The chief acknowledging neck restraints were permitted at the time but saying chauvin did not use reasonable force in keeping his knee on George Floyd's neck for over nine minutes. That in no way, shape or form is anything that is by policy. It's not part of our training, and it is certainly not part of our ethics or our values. Jimmy: Arredano fired chauvin. The chief's ereassessment damaging testimony from Richard Zimmerman, the most senior officer in the Minneapolis police department. In your view of that use of force during that time period? Totally unnecessary. The most significant evidence has been from fellow police officers who have said this was not reasonable force. This is not what our training tells you to do. That's very powerful testimony against Derek chauvin. Reporter: Chauvin is charged with second-degree unintentional murder, third-degree murder and second-degree manslaughter, accused of killing 46-year-old Floyd during the police stop. From chauvin's own body camera, you can hear his voice. Trying to control this guy because he is a sizable guy. Get in the car. He is probably on something. Reporter: Chauvin's defense attorney showing chief air Dando questioning if he was his knee was on Floyd's neck the entire time. Would you expect from the body camera that Mr. Chauvin's knee was more on Mr. Floyd's shoulder blade? Yes. Reporter: But chief arredando says Floyd may have already been dead. He noted that chauvin did not follow the department's policies on de-escalation or providing first aid. Still, prosecutors are facing the uphillel challenge of convicting a police officer who is accused of killing someone while on the job. Among officers charged in a fatal on-duty shooting, less than 50% have actually been convicted and often for a lesser since 2005, only seven officers have been convicted of murder. As a legal matter, police officers are allowed to do things that civilians can't do. And so the question becomes when do police officers cross that legal line. Reporter: Much of the prosecution's case rests on the video recording of chauvin's behavior. You can believe your eyes that it's a homicide. It's murder. You can believe your eyes. The video is the heart of the prosecution's case. You can see what was happening and what the result would be. Reporter: The power of police violence caught on tape through police body cams and bystanders' cell phone video has galvanized the public ever since Rodney king was beaten by Los Angeles police in 1991. In the criminal trial, four officers were found not guilty of excessive force. Not guilty of the crime of assault by force. Reporter: Srking widespread protests and violence. A year later, two officers were sentenced to 30 months in prison in a federal civil rights trial. Randy foster is a retired law enforcement officer who often testifies as an expert witness. People thought that the videotaped beating of Rodney king would be a game-changer, and yet it wasn't. At that particular moment, it wasn't, but if you look at it, fast forward to today, more civilians are taking their cell phone out to video police action. And police are responding to that trying to stop people from videoing. Reporter: Is it harder to get a conviction against a police officer than say a regular citizen? Yes. The public don't like to put their police behind bars. That's why video evidence is so important, because it kills the narrative of a lie. And I'm telling you this as a former law enforcement officer who investigated internal affairs. Yes, they will lie. They're just as human as me and Reporter: 70-year-old laquan McDonald was killed by Chicago police officer Jason Van Dyke in 2014. Police first told his family he was shot only once, but the family said they knew that wasn't true. I had a nephew that actually worked at the medical examiner's office. He was receiving his own cousin. So I get a phone call, they lie. This boy got a whole lot of bullet holes in him. Reporter: In 2015, a year after laquan's death, police released dash cam video showing Van Dyke getting out of his squad car, opening fire within six seconds and shooting laquan 16 times. How can you watch a young black human being be shot 16 times and not have any compassion or empathy? Reporter: His great uncle reverend Marvin hunter remembers laquan as a loving young man. He enjoyed making sure that other people were happy because he liked to keep people laughing. He kept things light-hearted. Reporter: The family set out on a crusade for justice. This is another immaterial moment, and we're going to have to do the heavy lifting to raise the question as to why black people cannot get justice in this country. Reporter: Charged with first-degree murder, Van Dyke's defense was that he feared for his life, firing his gun only after laquan ignored repeated demands to drop a three-inch knife during the incident. I was yelling at him to drop the knife. Reporter: Asked why if he felt threatened, why he was walking toward laquan. You thought you were backpedaling as you were firing shot after shot after shot. What I know now and what I thought at that time are two different things. Reporter: In a rare officer conviction, Van Dyke was found guilty in October of 2018 of the lesser charge of second-degree murder and 16 counts of aggravated battery with a firearm. His jury eight women and four only one African American. He was sentenced to six years and nine months in prison, but that's far less than what laquan's family had wanted. I felt betrayed. I felt angry because I knew that it was not enough. Reporter: Reverend hunter delivered a powerful victim impact statement in court, a message to Van Dyke written in the voice of his nephew. Why should this person who has ended my life forever be free when I'm dead forever? Reporter: Still, reverend hunter says his nephew's case helped set the stage for the trial of Derek chauvin. We did things that brought us to a place where for the first time in recent history, a police officer in uniform was convicted for a crime in which he had committed. Behold how good and pleasant it is. Reporter: Now reverend hunter urging George Floyd's family to hold steadfast in their quest for justice, hoping the Floyd family can get what his almost did, a sentence that reflects the crime. In the George Floyd case, the American justice system is on trial again, and she's fighting and searching for her soul.pshe came close with laquan. She can win with Floyd. And be sure to stay with ABC news live for complete coverage of the trial.
This transcript has been automatically generated and may not be 100% accurate.