The 19-year-old cashier who suspected George Floyd had handed him a counterfeit bill said Wednesday he still feels guilt for Floyd's death under the knee of a former Minneapolis police officer.

Christopher Martin took the stand on the third day of the criminal trial of Derek Chauvin, echoing other witnesses who expressed helplessness and regret about what they did and didn't do leading up to Floyd's killing in May 2020.

Martin is barely older than others who described for the jury their pain and shame as survivors. He testified that he felt "disbelief and guilt" that his brief interaction led to Floyd's death and the turmoil that followed it.

"If I would have just not taken the bill, this could have been avoided," said Martin, who was a cashier at Cup Foods, where the initial call to police was made that day.Martin soon stopped working at the store. He said he didn't feel safe.

Another witness called by the prosecution, Charles McMillian, 61, cried and heaved on the witness stand after watching police body camera footage of Floyd's arrest.

In the video, Floyd gasped that he was claustrophobic. He said he couldn't breathe and called for his mother. McMillian broke down.

Bystanders have testified about their horror and fear watching Floyd die on May 25, 2020. Their testimony -- along with the searing eyewitness videos -- are the backbone of the state's case. Prosecutors have asked jurors to focus on video -- the 9 minutes and 29 seconds that Chauvin knelt on Floyd's neck.

Defense attorney Eric Nelson has argued that the video does not fully capture the complexity of the moment. Chauvin was following his police training, the lawyer said. He argued that Floyd's cause of death was a combination of drug use and preexisting health issues. The crowds of bystanders distracted the officer, he said.

Chauvin, 45, has pleaded not guilty to charges of second-degree murder, third-degree murder and second-degree manslaughter.The case sparked a social reckoning with American racism and police brutality.

On Tuesday, a high schooler who recorded and shared video of Chauvin kneeling on Floyd said she had lost sleep thinking of what else she could have done.

"It's been nights I've stayed up apologizing to George Floyd for not doing more and not physically interacting and not saving his life. But it's not what I should have done, it's what he should have done," she said, referring to Chauvin.

Another witness was 9 years old.

"I was sad and kind of mad," the girl testified. "Because it felt like he was stopping his breathing, and it was kind of like hurting him."

Minneapolis firefighter and certified EMT Genevieve Hansen, who was out for a walk on her day off, told the jury she wanted to render aid to Floyd.

She said she repeatedly asked police to check for a pulse. They refused. She felt helpless."I tried calm reasoning, I tried to be assertive, I pled and was desperate," she testified.

"I was desperate to give help."During cross-examination by Nelson on Tuesday, Hansen took issue with his questioning.

"I don't know if you've ever seen someone die in front of you, but it's very upsetting," she said at one point.After dismissing the jury, Judge Peter Cahill admonished Hansen, telling her to answer questions and stop arguing.

CNN's Eric Levenson and Aaron Cooper contributed to this report.