From eyewitnesses to use-of-force and medical experts, along with police officers, 44 witnesses took the stand over three weeks of testimony in ex-Minneapolis police officer Derek Chauvin's trial for second-degree murder, third-degree murder and second-degree manslaughter in the death of George Floyd. The prosecution called 37 witnesses, while the defense called seven. Chauvin was convicted on all counts. Here are some key witnesses.

Donald Williams

Williams was a witness at the scene of 38th and Chicago and among the most vocal in urging Chauvin to get off Floyd, and for officer Tou Thao to intervene. Proficient in wrestling and mixed martial arts, he said Chauvin placed a "blood choke" on Floyd, restricting his circulation. "I called the police on the police," he testified. Asked why, he said, "Because I believe I witnessed a murder."

Genevieve Hansen

Hansen, a Minneapolis firefighter, was off-duty and out for a walk when she came upon the scene. She attempted to render aid to Floyd but was rebuffed by officers and told to stay back. Still, she repeatedly pleaded for them to check his pulse. "There is a man being killed, and I would have been able to provide medical attention to the best of my abilities, and this human was not provided that right."

Courteney Ross

George Floyd's girlfriend of three years. She met him at the Salvation Army Harbor Lights shelter where he worked as a security guard. She testified that they both struggled off and on with opioid addiction after both suffered chronic pain. "Both Floyd and I, our story is a classic story of how we both got addicted to opioids. … We got addicted and tried really hard to break that addiction many times."

Medaria Arradondo

Minneapolis police chief who testified of his department's training and policies, and how he learned of what happened the night Floyd was killed. "Once Mr. Floyd had stopped resisting — and certainly once he was in distress and trying to verbalize that — that should have stopped," the chief said after spelling out department policy on when to use force compared to using de-escalation tactics.

Dr. Martin Tobin

A physician in pulmonology and critical care in Chicago. He concluded that Floyd died from "a low level of oxygen. This caused damage to his brain that we see, and it also caused a [pulseless electrical activity] arrhythmia that caused his heart to stop." Floyd stopped breathing and "didn't have an ounce of oxygen in his body" less than a minute after losing consciousness, Tobin said.

Dr. Andrew Baker

Chief Hennepin County medical examiner who performed Floyd's autopsy and stood by his ruling that Floyd's death was a homicide caused by "cardiopulmonary arrest complicating law enforcement subdual, restraint and neck compression." The doctor tied in Floyd's "very severe underlying heart disease" and enlarged heart, combined with the stress of being pinned to the asphalt.

Barry Brodd

A use-of-force expert for the defense, Brodd testified that he believed Chauvin's actions were reasonable. Brodd, a former police officer in California, said it's important for anyone "to try to see it as the officer on the scene … then try to put yourself in the officer's shoes. It's easy to sit in an office and judge an officer's conduct." Brodd said he didn't believe the "prone control" position was use of force.

Dr. David Fowler

Fowler, an expert medical witness, recently retired as chief medical examiner for Maryland. He testified that Floyd died of cardiac arrest combined with illicit drugs in his body and not from a lack of oxygen. He would have classified Floyd's manner of death as undetermined due to the multiple factors he cited. The doctor said that carbon monoxide from the police squad could have been a factor.