A Minneapolis homicide detective has described Derek Chauvin’s decision to press his knee into George Floyd’s neck for more than nine minutes as a totally unnecessary use of “deadly force”. Lt Richard Zimmerman, who leads the Minneapolis homicide department, said in testimony on Friday that in four decades as a police officer he had never been trained to place a knee on someone’s neck as a means of restraining them during an arrest. “If your knee is on a person’s neck, that can kill them,” he said. In evidence likely to be highly damaging to the defence claim that Chauvin was acting out of concern for his own safety as he arrested Floyd, a 46-year-old Black man, in Minneapolis last May, Zimmerman described the level of force used by officers after Floyd was in handcuffs as “totally unnecessary”. “Once a person is cuffed, the threat level goes down all the way. They’re cuffed, how can they really hurt you?” he said. Video shows Floyd was already handcuffed by the time he was forced onto the ground. “I saw no reason why the officers felt they were in danger, if that’s what they felt. And that is what they have to feel to use that level of force,” said Zimmerman. Chauvin, 45, has denied charges of second- and third-degree murder, and manslaughter, over Floyd’s death. He faces up to 40 years in prison if convicted of the most serious charge. Three other officers face separate charges. Zimmerman was one of 14 police officers who published a public letter to the citizens of Minneapolis a month after Floyd’s death, to “wholeheartedly condemn” Chauvin’s actions. “Like us, Derek Chauvin took an oath to hold the sanctity of life most precious,” they wrote. “Derek Chauvin failed as a human and stripped George Floyd of his dignity and life. This is not who we are.” Zimmerman told the trial officers are trained in the dangers of keeping a detained person in the prone position on the ground. Floyd was held for more than nine minutes with Chauvin’s knee on his neck and two other officers pinning his torso and legs. “Once you secure or handcuff a person, you need to get them out of the prone position as soon as possible because it restricts their breathing,” Zimmerman said. Chauvin’s lawyer, Eric Nelson, put it to Zimmerman “that in a fight for your life, you as an officer are allowed to use whatever force is necessary?” Nelson challenged the detective’s claim never to have been trained to put a knee to a suspect’s neck, but Zimmerman said the training was to place the knee on a person’s shoulder and only while handcuffing them. Zimmerman also told the court that police failed to follow critical incident procedures intended to ensure officers are properly interrogated. Zimmerman said that procedure meant the officers involved in Floyd’s death should have been taken in separate squad cars to an interview room for questioning. Yet he found two of the officers, Alexander Kueng and Thomas Lane, still at the scene. He ordered them taken for questioning. Earlier, a Minneapolis police sergeant, Jon Edwards, who secured the scene after Floyd’s death, told the trial he found Kueng and Lane in their squad car together. He ordered them out and to turn their body cameras on to record their conversations. Zimmerman’s evidence followed similar testimony from Chauvin’s shift supervisor, Sgt David Pleoger, on Thursday. He said there was no justification for the officer to keep his knee on Floyd’s neck for nine minutes or for the other officers to continue to hold him down once he stopped resisting. “When Mr Floyd was no longer offering up any resistance to the officers they could have ended their restraint,” said Pleoger, who arrived at the scene shortly after Floyd was taken away by ambulance. He also said Floyd should not have been kept in the prone position because of the danger of “positional asphyxia”. “If they are left on chest or stomachs for too long, their breathing can be compromised,” he said. Pleoger said police officers are trained about the danger that a suspect can suffocate if they are not put on their side in the recovery position. The police evidence came at the end of a week that began with the prosecution putting witnesses on the stand who gave emotional evidence about Floyd’s struggle for life under Chauvin’s knee. Some of the most emotional testimony was heard on Thursday as Floyd’s girlfriend, Courteney Ross, spoke about their shared struggle with opioid addiction. Ross, who dated Floyd for about three years, said they both became hooked after being prescribed narcotics to treat chronic pain. Three other officers involved in Floyd’s death are scheduled to be tried together later this year on charges of aiding and abetting murder and manslaughter. The trial continues. Next week, the chief of the Minneapolis department, Medaria Arradondo, is expected to testify. In a highly unusual move, he will give evidence against his own former officer. Arradondo fired Chauvin shortly after Floyd’s death.