MINNEAPOLIS (NewsNation Now) — The emergency room doctor who pronounced George Floyd dead after trying to resuscitate him testified Monday that he theorized at the time that Floyd most likely died of suffocation.

Dr. Bradford Langenfeld, who was a senior resident on duty that night at Hennepin County Medical Center, took the stand at the beginning of the second week at former Minneapolis Officer Derek Chauvin’s murder trial, as prosecutors sought to establish that it was Chauvin’s knee on the Black man’s neck that killed him.

Langenfeld said Floyd’s heart had stopped by the time he arrived at the hospital. The doctor said that he was not told of any efforts at the scene by bystanders or police to resuscitate Floyd but that two paramedics told him they had tried for about 30 minutes.

Langenfeld told the jury he took over Floyd’s care. Asked by prosecutor Jerry Blackwell if the paramedics indicated that they suspected a drug overdose or heart attack, Langenfeld said they did not, indicating that only that Floyd’s heart had stopped beating and that there may have been a delay in starting resuscitation efforts.

Based on the information he had and medical tests, Langenfeld thought it was unlikely that Floyd suffered a heart attack, he told the jury. The most likely explanation, the doctor said, was asphyxia.

Death by asphyxiation, also described as oxygen deficiency, was “more likely than the other possibilities.”

Floyd was declared dead at 9:25 p.m., about 30 minutes after he arrived at the hospital and less than 90 minutes after police arrived outside the Cup Foods grocery store to arrest Floyd on suspicion of his using a fake $20 bill to buy cigarettes.

Chauvin, 45, is charged with murder and manslaughter in Floyd’s death. The white officer is accused of pinning his knee on the 46-year-old man’s neck for 9 minutes, 29 seconds, as Floyd lay face-down in handcuffs outside the corner market.

Floyd’s treatment by police was captured on widely seen bystander video that sparked protests that rocked Minneapolis and quickly spread to other U.S. cities and beyond and descended into violence in some cases.

The defense argues that Chauvin did what he was trained to do and that Floyd’s use of illegal drugs and his underlying health conditions caused his death.

Chauvin attorney Eric Nelson questioned Langenfeld about whether some drugs can cause hypoxia, or oxygen deprivation. The doctor acknowledged that fentanyl and methamphetamine, both of which were found in Floyd’s body, can do so.

The county medical examiner’s office ultimately classified Floyd’s death a homicide — that is, a death at the hands of someone else.

The full report said Floyd died of “cardiopulmonary arrest, complicating law enforcement subdual, restraint, and neck compression.” A summary report listed fentanyl intoxication and recent methamphetamine use under “other significant conditions” but not under “cause of death.”

Under cross-examination from Nelson, Langenfeld said Floyd’s carbon dioxide levels were more than twice as high as levels in a healthy person, and he agreed that that could be attributed to a respiratory problem. But on questioning from the prosecutor, the doctor said the high levels were also consistent with cardiac arrest — the stopping of the heart.

Langenfeld also testified that neither he nor paramedics administered a drug that would reverse the effects of an opioid overdose. The doctor said giving Narcan once a patient is in cardiac arrest would provide no benefit.

He said there is an approximately 10% to 15% decrease in survival for every minute that CPR is not administered.

“It’s well known that any amount of time that a patient spends in cardiac arrest without immediate CPR markedly decreases the chance of a good outcome,” he told the jury.

The Associated Press and Reuters contributed to this report. Reporting by Amy Forliti/AP, Tammy Webber/AP and Jonathan Allen/Reuters.