(CNN) An unprecedented lineup of law enforcement officers -- including the Minneapolis police chief -- took the stand at the murder trial of former officer Derek Chauvin, denouncing him for kneeling on the neck of George Floyd for more than nine minutes.
As rare as it is for police-involved deaths to lead to a criminal trial -- let alone a conviction -- the sight of the top cop and other brass testifying against a former subordinate is even rarer.
Chief Medaria Arradondo on Monday thoroughly rejected Chauvin's actions and use of force during the arrest of Floyd last May as contrary to department policy.
"Once there was no longer any resistance and clearly, when Mr. Floyd was no longer responsive and even motionless, to continue to apply that level of force to a person proned out, handcuffed behind their back -- that in no way shape or form is anything that is by policy," the police chief told the jury. "It is not part of our training, and it is certainly not part of our ethics or our values."
The piercing of the so-called blue wall of silence -- an unwritten code that has long restrained cops from implicating fellow officers accused of misconduct -- in one Minneapolis courtroom has been widely praised following a summer of protest, unrest and a social reckoning with American systemic racism.
"It is unprecedented," Baltimore City State's Attorney Marilyn Mosby said. "You have police officers. You have training officers. You have the police chief who is willing to testify in direct opposition to Chauvin's extensive use of force. That does not happen every day."
Still, other experts caution that the potentially devastating testimony in a nationally televised trial does not necessarily portend significant change in longstanding systemic problems in US policing -- particularly the way people of color are treated.
"It's absolutely too soon to say whether we are really entering a new era where police chiefs and others not only refuse to cover up for officers' conduct but actually ... come forward and speak very powerfully and very unequivocally about officer misconduct the way that we're seeing in the Chauvin trial," said Christy Lopez, a Georgetown Law professor and former Justice Department lawyer who investigated police departments.
"We've definitely got some momentum now and that could really build into something permanent but it could end up being a flash in the pan if we don't take advantage of this moment."
Chauvin was fired from his position as a Minneapolis police officer soon after a cellphone video went viral of him planting his knee on Floyd's neck. Floyd had been accused of passing a counterfeit $20 bill. He is heard repeatedly crying out for help. He said he couldn't breathe, and called for his mother. Three other former officers were present.
Chauvin knelt on Floyd for nine minutes and 29 seconds, according to prosecutors. A renowned pulmonary critical care doctor testified Thursday that Floyd died from a "low level of oxygen" when Chauvin pinned him to the street and restricted his ability to breathe.
Less than a year after Floyd's death, Chauvin is the first of the four former officers at the scene on May 25 to go to trial. He faces the more serious charges.
Chauvin, 45, has pleaded not guilty to second-degree unintentional murder, third-degree murder and second-degree manslaughter. Chauvin's defense attorney Eric Nelson has argued that Floyd died of a drug overdose and preexisting health conditions.
Hennepin County Chief Medical Examiner Dr. Andrew Baker testified Friday that Floyd's heart disease and use of fentanyl were contributing factors but not the direct cause of his death.
"In my opinion, the law enforcement subdual, restraint and the neck compression was just more than Mr. Floyd could take by virtue of those heart conditions," he said.
The defense has also argued that Chauvin acted within his police training and employed an appropriate use of force.
Ramsey County Attorney John Choi, whose office prosecuted the Minnesota police officer who fatally shot Philando Castile during a 2016 traffic stop, said the number of officers who have testified against Chauvin is indeed unusual.
Jeronimo Yanez, the St. Anthony, Minnesota, police officer, was found not guilty of second-degree manslaughter in 2017 after the police chief, training officers and other colleagues testified in Yanez's defense. "They were trying to help their colleague who was accused of a crime," Choi said.
The jury acquitted Yanez despite video footage and testimony from Castile's girlfriend, who witnessed the shooting. Castile's death garnered widespread attention -- and sparked nationwide protests -- after his girlfriend broadcast the shooting's aftermath on Facebook Live.
An audio recording captured Castile telling Yanez he had a gun in the car, and the officer telling Castile not to reach for it. Seconds later, Yanez opened fire. Yanez testified at trial that he feared for his life because Castile put his hand on his firearm, not his wallet or identification papers, and was pulling the gun from his pocket. Prosecutors said Castile's fully loaded gun -- for which he had a permit -- was found in his shorts pocket.
Miriam Krinsky, a former federal prosecutor who led the appeal on the lenient sentencing of one of the officers involved in the 1991 beating of Rodney King, said there is "no such thing as a slam dunk case when it comes to prosecuting police officers ... even with a videotape."
"By human nature people on juries are just inclined to give police officers the benefit of the doubt and in every way, shape or form to individuals who wear a uniform," Krinsky said. "Jurors are inclined to want to trust police because these are people who are in charge with keeping them safe in their homes and in their neighborhoods."
How the jury in Chauvin's closely watched trial is swayed by the testimony of a succession of officers testifying for the state remains to be seen.
Arradondo, who joined the Minneapolis Police Department in 1989 and rose through the ranks, said the former officer's handling of the encounter violated policies around de-escalation, objectively reasonable use of force and requirement to render aid.
"When we talk about the framework of our sanctity of life and when we talk about the principles and values we have, that action goes contrary to what we're talking about," he said.
Arradondo had previously testified at the 2019 trial of Mohamed Noor, an officer convicted of third-degree murder and manslaughter for fatally shooting Justine Ruszczyk while responding to her 911 call. Arradondo was assistant chief at the time of the shooting.
Noor was believed to be the first officer convicted of murder in Minnesota in recent memory, according to prosecutors.
In addition to Arradondo, Police Inspector Katie Blackwell -- who was in charge of the department's training program last year -- testified in Chauvin's trial on Monday that officers are taught to use their arms when doing neck restraints.
"I don't know what kind of improvised position this is," she said of Chauvin's kneeling. "That's not what we train."
Chauvin's direct supervisor testified that the ex-cop's use of force should have ended earlier, and the department's top homicide detective testified that kneeling on Floyd's neck after he had been handcuffed was "totally unnecessary."
"The thing that people forget is that there are a lot of prosecutions of individual officers in which other officers ... as well as chiefs really put their careers on the line and put their safety on the line to testify against those officers," Lopez said.
"It's worth recognizing and sort of honoring the officers who have come forward in similar situations in the past and have really paid a price for it," she added. "If we want this to be more than a moment that's the dynamic we have to change."
At least one defense attorney with experience representing police officers dismissed the significance of back-to-back law enforcement testimony in the Minneapolis trial.
The attorney, who didn't want to be identified because of the sensitivity of the ongoing trial, said the police testimony "doesn't matter" because the witnesses are not considered experts and are offering "just an opinion and opinions don't matter in trial."
"These cases are supposed to be a battle of the experts on testimony to say whether this death was caused by force or medical," the defense attorney said. "I think they're trying to sort of bootstrap the experts they're going to have with other credentialed people."
In Chicago, Cook County State's Attorney Kimberly Foxx singled out the testimony of the first witness called in Chauvin's trial, Jena Scurry, a Minneapolis 911 dispatcher who directed officers to the scene of Floyd's death.
Scurry walked jurors through video, not previously publicly released, shot from a police camera across the street from the Cup Foods store where police were first called. She watched live video from that feed on the day of Floyd's death and called a police sergeant to voice her concerns.
"You can call me a snitch if you want to," she said in the recorded call. "I don't know if they had to use force or not. They got something out of the back of the squad and all of them sat on this man, so I don't know if they needed to or not."
Foxx noted how "that was in real time and in real time you had people saying, 'This is egregious.' "
"I think the testimony has proven that in real time other than those four officers, others in law enforcement were disgusted, outraged and recognized the criminality of this."
Lopez, co-director of Georgetown Law's Program on Innovative Policing, said the program is leading a nationwide initiative known as Active Bystandership for Law Enforcement Project, or ABLE. It is training officers in more than 100 US law enforcement agencies to step in when another officer uses excessive force or acts against department guidelines.
The project has become popular with police agencies following the protests that erupted after Floyd's death. Lopez said there is an advantage to dealing with systemic issues in policing at the reform level rather than the prosecution level after "something terrible has already happened."
Intervening is a much more effective way to prevent fellow officers from causing harm than trying to chip away at the blue wall of silence.
"That's what was missing (in the Floyd police encounter) -- both the culture and the skills for these fellow officers to step in and actually, first, tell Chauvin, you need to get your knee out of his neck and then, if necessary, physically remove him from George Floyd's neck," Lopez said.
She added, "You could so clearly see on the video that these officers didn't intervene and how different things could be if they had... I don't want to discount the importance of accountability. But there's no question in my mind that this kind of broad training and culture change that is geared towards prevention is going to make a much bigger difference than after-the-fact prosecutions."
Mosby said the series of law enforcement witnesses in Chauvin's nationally televised trial "allows for other police officers to say, 'I can do the right thing.' "
"We have normalized the code of silence," she added. "That is the culture."
Mosby charged six Baltimore Police Department officers in connection with the 2015 death of Freddie Gray, 25, who died in police custody from a spinal cord injury. The death led to days of protests and riots in Baltimore, which became a focal point of the Black Lives Matter movement.
After the first officer's trial ended with a hung jury -- one juror held out -- three other officers went on to have non-jury trials where a judge acquitted them. Mosby's office later dropped the charges against the officer whose trial ended in a hung jury and the two remaining officers. Mosby at the time called the decision "agonizing." She criticized the way police handled their investigation in the case.
David Rudovsky, a civil rights and criminal defense attorney who teaches at the University of Pennsylvania Carey Law School, cautioned against interpreting the Chauvin trial so far as an example of "a major change in American policing."
"I still don't expect that we're going to see a lot of prosecutions and a lot of convictions of police," he said. "I think it's a necessary tool (but) police reform is much more important, is more likely to happen with internal accountability. The external is important: criminal prosecution, investigations, civil suits. That helps but internal accountability is really the key."
Still, some see Chauvin's prosecution as an important step.
Civil rights leaders expressed surprise that top ranking police officials cracked the blue wall of silence in the Chauvin trial. It's especially rare, they said, in police brutality cases where the victim is Black.
NAACP President Derrick Johnson said the police chief's decision to immediately fire Chauvin and then testify against him sets the example for how law enforcement can begin to weed out rogue officers.
"We hope their testimony is as compelling to the jury as it is to us," Johnson said of the police witnesses. "I don't see how there is any reasonable doubt."
But Rashad Robinson, president of Color of Change, a racial justice organization, said he isn't convinced the testimony will weigh heavily on the jury's final decision or alter how police treat Black people.
Robinson believes Minneapolis police are using a "bad apple strategy" to separate the department from Chauvin rather than addressing the systemic issues fueling brutality by officers.
"Derek Chauvin saw cameras in his face and did not flinch because this is policing in America," he said.
Lopez said acknowledging the significance of the trial testimony is important -- without making too much of it.
"This moment is an opportunity. This moment reflects potential. There's some momentum being built here but we just know from history and from what we know about policing that this will subside," she said.
"Nothing or very little will change unless we really think about what are the lessons of this moment. How can we encourage this kind of accountability, this kind of standing up for what's right? Not just when you're on national TV and cities have burned, but every single day in policing."
Correction: A previous version of this story misstated the charges Derek Chauvin faces in George Floyd's death. They are second-degree unintentional murder, third-degree murder and second-degree manslaughter. He has pleaded not guilty.