New York (CNN) A defense lawyer and a New York City police union have cast doubt on her findings that an officer's chokehold led to Eric Garner's death, but the medical examiner stood by her conclusion during Wednesday testimony.

Under questioning from Suzanne O'Hare of the city's Civilian Complaint Review Board, which is acting as prosecution in the proceedings, medical examiner Floriana Persechino said Officer Daniel Pantaleo caused Garner to have an asthma attack when he put the 43-year-old in a chokehold during a 2014 arrest.

The chokehold, she said, was "part of the lethal cascade of events."

Explaining that "application of pressure to the anterior neck is a painful phenomenon," Persechino said Garner's neck displayed fresh hemorrhaging when she examined him.

"In my opinion, the hemorrhaging was due to compression of the neck by the chokehold," she said.

Shown footage of the arrest, in which Pantaleo wraps his forearm around Garner, she said, "In my opinion, that's a chokehold."

In the days after Garner's death, Persechino's office announced Garner's cause of his death was "compression of neck (choke hold), compression of chest and prone positioning during physical restraint by police." Police said he had suffered a heart attack on the way to the hospital.

Union, attorney have challenged findings

Pantaleo's attorney has previously called the autopsy report "wrong" and Persechino's findings "inaccurate." He has blamed the 350-pound man's death on obesity and other health issues. Those assertions have been echoed by the New York City Police Benevolent Association and its president.

"He was a ticking time bomb and set these facts in motion by resisting arrest," Pantaleo's lawyer, Stuart London, has said.

While London pointed out during cross-examination that the larynx and neck cartilage were not damaged, Persechino said that was not necessary to prove a chokehold was applied.

London also highlighted how the whole encounter didn't last very long, only a few seconds, and how it's unclear how much pressure was applied.

"You have no idea how much pressure?" asked London.

"I cannot quantify how much pressure, no," the medical examiner said.

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The medical examiner's testimony comes a day after Inspector Richard Dee, the commanding officer for the New York police's training section, saw a still from the arrest video and testified that the move meets the definition of a chokehold.

Whether Pantaleo administered a chokehold -- long banned by the New York Police Department -- is key to the disciplinary proceedings. Though video shows Pantaeo put his arm around Garner's neck as officers take him down, London has characterized the technique as a "seatbelt maneuver" that is in line with police training.

Pantaleo never obstructed Garner's breathing, the defense lawyer has said.

Garner's mother walks out again

Garner's mother, Gwen Carr, has been attending the proceedings. She left the hearing room Wednesday upon learning that lawyers intended to show graphic photos from her son's autopsy, including images of hemorrhaging to his face and neck.

Carr left the room or averted her eyes earlier this week when video of the arrest was shown. She said she's never seen it in its entirety.

In a Tuesday statement, she said it was difficult to sit near Pantaleo, and she accused the defense of twisting the facts in an attempt to vindicate the officer.

"lt's very, very hard on me," she said. "To watch these tapes over and over again -- and then to watch the defense with their manipulations. They manipulate the answers. They try to manipulate the witnesses. It's so unfair. ... They want to prove that it wasn't a chokehold, when the chief inspectors are saying it's a chokehold."

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Carr has called for Pantaleo to be fired and for disciplinary proceedings to be brought against the other officers involved in her son's arrest.

The New York Police Department is expected to launch disciplinary proceedings against Pantaleo's supervisor, Sgt. Kizzy Adonis, one of the first officers to respond to the scene.

Wednesday marked day three of a 10-day disciplinary hearing that will determine whether Pantaleo can remain on the force.

Rosemarie Maldonado, the department's deputy commissioner for trials, is overseeing the proceeding. If Pantaleo is found guilty of using a banned chokehold, Maldonado can recommend he be terminated. Commissioner James O'Neill would then determine whether Pantaleo can keep his job.

After waiting out a US Justice Department investigation into the arrest, which remains ongoing, New York police in July moved forward with departmental charges of using a chokehold and restricting Garner's breathing.

'I can't breathe'

It's been almost five years since Garner died after telling officers, "I can't breathe" -- words that became a rallying cry against police use of excessive force.

On July 17, 2014, Garner was allegedly selling "loosies," or single cigarettes -- something he had been arrested for in the past -- in the Tompkinsville neighborhood of Staten Island.

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Camera phone footage shows Garner arguing with two officers, denying he's selling cigarettes and repeatedly asking to whom he is accused of selling them. He's upset, gesticulating as he accuses officers of previous harassment.

As several officers take him into custody, Pantaleo puts his arm around Garner's neck and the officers take Garner to the ground seconds later. Pantaleo forces Garner's head into the sidewalk, eliciting Garner's muffled cries of "I can't breathe. I can't breathe."

The medical examiner said Garner had an enlarged heart, was obese with a BMI (body mass index) over 50% and suffered from hypertension.

Carr and activists were critical of Mayor Bill de Blasio and the city because Pantaleo has not been fired.

"I know that I'll never be able to see those photos of my child that they had to dissect because a man had murdered him," Carr told reporters after the hearing.

"No way I could sit in that room and view the photos of my child," she said.

Testimony will begin Thursday from Pantaleo's defense team. They will highlight what Pantaleo was doing in the area and why he responded.