NEW YORK (AP) — When Daniel Penny fatally choked a homeless man aboard a Manhattan subway last year, the 25-year-old veteran appeared to be using a combat technique that he learned in the U.S. Marines, according to the martial arts instructor who served alongside Penny and trained him in several chokeholds.

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

“Once the person is rendered unconscious, that’s when you’re supposed to let go,” Caballer said.

His testimony came weeks into the trial of Penny, who faces manslaughter charges after placing Jordan Neely, a homeless man and Michael Jackson impersonator, in the fatal chokehold last May.

Neely, who struggled with mental illness and drug use, was making aggressive and distressing 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. Marines.

Bystander video showed Penny with his bicep pressed across Neely’s neck and his other arm on top of his head, a position he held for close to six minutes, even after the man went limp.

The technique — an apparent attempt at a “blood choke” — is taught to Marines as a method to subdue, but not to kill, an aggressor in short order, Caballer said. Asked by prosecutors if Penny would have known that constricting a person’s air flow for that length of time could be deadly, Caballer replied: “Yes.’”

“Usually before we do chokes, it’s like, ‘Hey guys, this is the reason why you don’t want to keep holding on, this can result in actual 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.”

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

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

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

Racial justice protesters have appeared almost daily outside the Manhattan courthouse, labeling Penny, who is white, a racist vigilante who overreacted to a Black man in the throes of a mental health episode.

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

Following Neely’s death, U.S. Rep. U.S. Matt Gaetz, who President-elect Donald Trump nominated this week as his Attorney General, described Penny on the social platform X as a “Subway Superman.”