Who were the key witnesses who helped determine the outcome in Derek Chauvin trial?

Arguments in the trial of Derek Chauvin, who has now been convicted of the murder of George Floyd, largely focused on two key questions: What specifically was the cause of Mr Floyd’s death, and were Chauvin’s actions “reasonable” given the circumstances and his training?

As the former Minneapolis police officer stood trial, a total of 45 witnesses worked to answer these questions. Out of so much compelling testimony, two key witnesses stood out: Dr Martin Tobin for the prosecution, and Barry Brodd, a former California police officer, for the defence.

But the most important witness of all was a teenager named Darnella Frazier – not because of her testimony, but because of the evidence she provided: the infamous video of Mr Chauvin leaning his knee on Mr Floyd’s neck.

Darnella Frazier

Ms Frazier was 17 when she filmed the video that shook the world. Outside of a Cup Foods grocery store in Minneapolis, she used her cell phone to record the disturbing spectacle of three police officers pinning a man to the street.

As the victim, George Floyd, cried out in pain and bystanders shouted at the police to stop, Mr Chauvin refused to budge. His knee remained on Mr Floyd’s neck for over 9 minutes, until he was dead.

Ms Frazier posted the video to Facebook, where it quickly went viral. The footage contradicted the police’s original account of Mr Floyd’s death, which vaguely said Mr Floyd was “suffering medical distress” and then died. It neglected to mention the knee on his neck.

As the video spread, protests erupted all over the United States and around the world. Riots broke out in Minneapolis, where a police precinct was burned down. Former president Trump hid in a bunker below the White House as protesters raged outside. Americans, for the first time in large numbers, debated whether to “defund the police.”

All this happened long before Ms Frazier was called to the stand at Mr Chauvin’s trial. But when she did testify, her words were painful and moving.

“It’s been nights I stayed up apologizing and apologizing to George Floyd for not doing more and not physically interacting and not saving his life,” Ms Frazier said in court.

“When I look at George Floyd, I look at my dad, I look at my brothers, I look at my cousins, my uncles, because they’re all Black,” she went on. “I look at how that could have been one of them.”

After Mr Chauvin was convicted on Monday, Ms Frazier, now 18, wrote words of celebration.

“I just cried so hard,” she wrote on Instagram. “This last hour my heart was beating so fast, I was so anxious, anxiety bussing [sic] through the roof. But to know GUILTY ON ALL 3 CHARGES!!!”

“George Floyd we did it!!” she added. “Justice has been served.”

Dr Martin Tobin

Dr Tobin, a lung expert and ICU doctor, testified that George Floyd died because his lungs weren’t able to get enough air, impairing the brain and causing his heart to stop.

“Mr Floyd died from a low level of oxygen,” he said when called to the stand by the state. “The cause of the low level of oxygen was shallow breathing, small breaths, small tidal volume, shallow breaths that weren’t able to carry the air through his lungs.”

Four factors caused this lack of oxygen, according to the testimony: Mr Floyd was in a prone position, he was in handcuffs against hard pavement, there was a knee on his neck, and that there was a knee on his back. Together these forces cut off practically all function in his left lung, according to Dr Tobin.

“Basically on the left side of his lung, it was almost like a surgical pneumonectomy. It was almost like a surgeon had gone in and removed the lung,” the medical expert said.

“It’s like the left side is in a vice. It’s totally being totally pushed in, squeezed in from each side,” he added.

In addition to compressing Mr Floyd’s lungs, Mr Chauvin also reduced the amount of air that could come in through a passage in the bottom of the throat called the hypopharynx by kneeling on his neck, the lung doctor testified.

According to the doctor’s testimony, Mr Chauvin had an estimated ninety pounds of pressure on Mr Floyd’s neck at times.

Dr Tobin also suggested that Mr Floyd may have suffered a brain injury about five minutes into the police officer kneeling on top of him. In video evidence from the day, it was at this point that Mr Floyd ceased to speak and cry out.

“You can’t speak without a brain being active, so we know there’s oxygen getting to his brain whenever he is making an attempt to speak,” Dr Tobin said.

Dr Tobin’s testimony was the most important part in the state’s case to confirm the cause of Mr Floyd’s death was that former Minneapolis police officer Chauvin smothered him when he kept his knee on his neck for nine minutes.

The defence, meanwhile, suggested that a drug overdose and Mr Floyd’s pre-existing heart condition were responsible.

However, according to Dr Tobin, the fentanyl in Mr Floyd’s body at the time didn’t play a role in slowing his breathing until he ultimately died. Fentanyl is a powerful opioid that can slow the breathing rate, but Mr Floyd continued to take breaths at a normal rate before passing out about five minutes into his detention on the ground.

“It tells you that there isn’t fentanyl on board that is affecting his respiratory centres,” Dr Tobin said.

Barry Brodd

For the defense, the most important witness was police trainer and use-of-force expert Barry Brodd, who testified that Derek Chauvin was “justified” when he put his knee on George Floyd’s neck for more than nine minutes.

“I felt that officer Chauvin’s interactions with Mr Floyd were following his training, following current practices in policing, and were objectively reasonable,” he said.

Mr Brodd was Mr Chauvin’s legal team’s first major witness, and the first to offer this positive interpretation of Mr Chauvin’s actions. Numerous senior Minneapolis police officers, and an independent use of force expert, had argued that the former officer went way overboard, violated proper police protocols, and failed to properly carry out his legal duty to medically care for someone in his custody.

Contrary to these opinions, Mr Brodd testified that the “prone control” position, pressing someone against the ground to handcuff or otherwise overpower them, was not inherently a use of force because it was unlikely to cause pain, though he conceded under questioning it was possible.

“It doesn’t hurt,” he said. “You’ve put the suspect where it’s safe for you, the officer, safe for the suspect, and you’re using minimal effort to keep them on the ground.”

But under questioning from the prosecution, Mr Brodd’s testimony began to hurt Mr Chauvin more than it helped.

At a crucial moment, he conceded that restraining someone in the prone “control” position as they’re being handcuffed could cause pain, which would amount to unjustified force if inflicted on a compliant person.

“If someone is not resisting, and they’re compliant, the use of ‘control,’ as you put it, that could produce pain is just not justified, is it?” prosecutor Steve Shleicher asked him.

“No,” Mr Brodd responded.