The police officers who pummeled Kelly Thomas during a violent encounter last summer in Fullerton caused his death by cutting off the flow of oxygen to his brain when the fight intensified and they piled on the homeless man, a coroner’s pathologist testified Tuesday.

Dr. Aruna Singhania, who told the court she had performed 11,000 autopsies, said the difficulty Thomas had breathing because of chest compression as the struggle wore on was worsened by facial and nasal bleeding.

The testimony came in the second day of a preliminary hearing that has orbited around a graphic and disturbing video of Thomas’ being hit by police outside the bus depot in downtown Fullerton.

On Tuesday, the back-and-forth in the courtroom centered less on the violence captured on video than on the medical question of what — and ultimately who — killed Thomas.

Singhania showed graphic images of Thomas’ body, pointing out bruises and wounds, and said the chest compression from the weight and position of the officers is what caused him to suffer hypoxia, a lack of oxygen in his blood.

The pathologist said Thomas ultimately died of brain death, acute bronchial pneumonia and blunt force facial injuries.

As images of Thomas’ battered and heavily bruised body resting on a coroner’s slab were about to be shown, his father, Ron Thomas, stood up and walked out of court. He said he was unable to look at the gruesome photos.

The reaction mirrored the response Monday when some spectators left the courtroom as the video played. The judge, after some in the audience groaned, stopped the tape at one point and asked those who could not stomach the images to leave the courtroom.

Thomas, who suffered shattered bones in his face, broken ribs and bruises over much of his body, was taken off life support by his family and died five days after his encounter with police.

Officer Manuel Ramos, 38, is charged with second-degree murder and involuntary manslaughter. Cpl. Jay Cicinelli, 40, is charged with involuntary manslaughter and using excessive force. Ramos faces a possible life prison term; Cicinelli four years in prison. Both have pleaded not guilty.

When the images of Thomas were flashed on the screen, Cicinelli didn’t look up. Ramos glanced at the photos, but only for a moment.

Prosecutors maintain that Ramos escalated what should have been a routine police encounter into a deadly beating by threatening Thomas, first saying, “See my fists,” and then adding “I’m getting ready to f— you up.”

During the scrum, Cicinelli can be seen on the video striking Thomas several times in the face with the butt of a Taser stun gun as officers pin him on the ground. “I just smashed his face to hell,” he can be heard telling a fellow officer.

Defense attorneys sought to upend the case by suggesting that emergency medical workers — not the police officers — caused his death by failing to promptly get tubes into Thomas’ airway.

Ramos’ attorney, John Barnett, tried to get the pathologist to pinpoint when and where Thomas’ chest was compressed to the degree that oxygen was cut off.

“It is not the heavy weight, it is the way it is compressed,” she said, pointing to bruising on Thomas’ left side.

She eventually agreed that the chest compression happened when Thomas’ cries for help could no longer be heard on the tape.

Barnett noted that his client was on Thomas’ legs — not his chest — at that time.

richard.winton@latimes.com