The sixth day of the trial of Derek Chauvin, the former police officer accused of killing George Floyd, brought two key witnesses to the stand: the doctor who spent 30 minutes trying to save Mr. Floyd’s life before pronouncing him dead, and the chief of the Minneapolis Police Department.

Both witnesses provided testimony that could bolster the arguments of the prosecution, which has argued that Mr. Floyd died because Mr. Chauvin knelt on him for more than nine minutes, rather than by complications of drug use or a heart condition. Here are the key takeaways from Monday.

Dr. Bradford T. Wankhede Langenfeld, who was a senior resident at the Hennepin County Medical Center, said he believed that Mr. Floyd died from a lack of oxygen. Mr. Floyd’s cause of death will prove to be a determining factor in this case. The prosecution has maintained that “asphyxia,” or a deficiency of oxygen, caused Mr. Floyd’s death. During a cross-examination, Dr. Wankhede Langenfeld told Eric J. Nelson, Mr. Chauvin’s lawyer, that asphyxia can be caused by a number of factors, including drug use; a toxicology report found methamphetamine and fentanyl in Mr. Floyd’s system.