NEW YORK (AP) — When Daniel Penny choked to death a homeless man on a Manhattan subway train last year, the 25-year veteran appeared to be using a fighting technique he learned in the U.S. Marines, according to the instructor of martial arts who served alongside Penny and coached him in several chokes.

But contrary to training, Penny maintained her grip around the man’s neck after he appeared to lose consciousness, turning the non-lethal maneuver into a potentially deadly choke, instructor Joseph Caballer testified Thursday.

“Once the person is unconscious, that’s when you should let go,” Caballer said.

His testimony came weeks after the trial of Penny, who faces manslaughter charges after fatally choking Jordan Neely, a homeless man and Michael Jackson impersonator, last May.

Neely, who has struggled with mental illness and drug use, was making aggressive and upsetting comments to other riders when he was taken to the ground by Penny, a Long Island resident who served four years in the U.S. Navy.

Viewer video showed Penny with her bicep pressed over Neely’s neck and her other arm over the top of his head, a position she held for nearly six minutes, even after the man went limp.

The technique — an apparent “blood drowning” attempt — is taught to Marines as a method to quickly subdue, but not kill, an aggressor, Caballer said. Asked by prosecutors if Penny knew that restricting a person’s airflow for that amount of time could be fatal, Caballer said, “Yes.”

“Usually before we choke, it’s like, ‘Hey guys, this is why you don’t want to keep holding, this can lead to real injury or death,'” the witness said. Being placed in such a position for even a few seconds, he added, “feels like trying to breathe through a crushed straw.”

Penny’s attorneys argue that their client sought to restrain Neely by placing him in a headlock, but that he did not apply strong force throughout the interaction. They raised doubts about the city medical examiner’s finding that Neely died of suffocation, pointing to his health problems and drug use as possible factors.

In his cross-examination, Caballer admitted that he could not “definitely state, watching the video, how much pressure is actually being applied.” But at times, he said, it looked like Penny was trying to restrict the flow of air to the blood vessels in Neely’s neck, “maybe cutting off one of the carotid arteries.”

Caballer is one of the final witnesses prosecutors are expected to call in a trial that has divided New Yorkers as it casts a national spotlight on the city’s response to crime and disorder in its transit system.

Racial justice protesters showed up almost daily outside the Manhattan courthouse, labeling Penny, who is white, a racist justice who overreacted to a black man in the throes of a mental health episode.

But he was also embraced by conservatives as a good Samaritan who used his military training to protect his fellow riders.

After Neely’s death, U.S. Rep. Matt Gaetz, whom President-elect Donald Trump nominated this week to be attorney general, described Penny on social media platform X as a “Subway Superman.”

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